Can't remember the exact size, it may have been around one inch in diameter. I may have to go back to my friend's yard and pick some leaves myself rather than relying on her; I hope that she didn't make a mistake (sorry about that).
If it helps, here is another view of the gall

I debated among Quercus palustris, Q. velutina, Q. ilicifolia, and others (read on). Two oak experts with whom I shared the photo suggested Q. palustris, but both of them indicated that they were not certain of that ID. I am open to other suggestions!

I see on the BG Info Tab for Amphibolips quercusinanis that Red Oak (Q. rubra), Scarlet Oak (Q. coccinea), and "Spanish Oak" are listed as possible hosts. The latter common name is a bit problematic because it has been applied to five species in the eastern U.S.; even limiting it to species whose natural range includes Pennsylvania (assuming that this suburban garden tree is indeed planted within its native range) leaves Q. palustris, Q. coccinea, and Q. falcata; given that Felt mentioned Q. coccinea separately (and further assuming he did not mean Q. palustris), I will assume he must have meant Q. falcata as Spanish Oak. I considered each of these prior to gravitating to the ones mentioned in my first paragraph because the symmetrical leaf shape (with U-shaped sinuses) in Beatriz's photo does not neatly fit Q. rubra (which is obviously broadest above the middle), Q. coccinea (which has multiple teeth and bristles per lobe, and C-shaped sinuses), or Q. falcata (obviously broadest below the middle). Of these three, Q. coccinea comes the closest. It may well be that the leaf specimens in Beatriz's photo are not typical due to tree age, leaf age, shading, hybridization, or some other factor.

That said, I agree that the gall itself better fits Amphibolips quercusinanis than A. confluenta/quercusspongifica. In light of that, likely oak candidates would be Q. coccinea or Q. rubra with the caveat that these leaves are not "textbook examples." Sorry for this long explanation without an unambiguous botanical conclusion, but this is how it goes with oaks sometimes!

Fortunately, Felt provides a key to his oak common names:
Spanish = Q. rubra
Red = Q. maxima.
Good to know--I'll fix that on the guide page.
If this gall was large (an inch or so), I think it would be safe to call it Amphibolips quercusinanis.

Don't mean to hijack this post as a forum for oak taxonomy, but to follow up on those names:

Q. rubra is commonly known today as "Red Oak" or "Northern Red Oak" ("Spanish Oak" applied to this species these days would puzzle botanists and foresters)

Q. maxima is an obsolete scientific name for Q. rubra, more specifically Q.r. var. rubra.

why I've never heard of Q. maxima, and it's good to know that Felt's "red oak" is the same species as what I would call red oak. I don't know why I'd never looked into this before; I guess I just assumed Spanish oak was some introduced ornamental in the red oak group.

During my career, I have never known anyone to use the name "Spanish Oak" (perhaps I have stayed too far north). I have always wondered what it meant, so this discussion prompted me to conduct a quick evaluation by consulting three main sources. The Silvics of Forest Trees of the United States applies this name (among others) to Q. falcata, Q. coccinea, and Q. palustris. The Field Guide to Native Oak Species of Eastern North America cites those three plus Q. buckleyi and Q. virginiana. Perusal of the internet (apply "grain of salt" rule) via Google Search repeats those five plus Q. texana (often confounded with Q. buckleyi) and Q. rubra.

Thus the three species are consistently called Spanish oak (as one of their common names) among all three sources are Q. falcata (also known as Southern Red Oak), Q. coccinea (also known as Scarlet Oak), and Q. palustris (also known as Swamp Oak or Swamp Spanish Oak). Spanish Oak was applied to Q. rubra the fewest times, only on the internet (including Wikipedia, which provided no additional background information).

It seems odd to me that Felt would refer to the same species by two names in the same appendix (Q. maxima = Red Oak = Q. rubra). I wonder if there was confusion during editing of that appendix among "Red Oak (Q. rubra)" and "Southern Red Oak/Spanish Oak (Q. falcata)?

If Q. maxima refers to var. rubra, might Q. rubra refer to another variety? I do feel like Spanish oak is meant to refer to Q. rubra, given that it is listed as a host for Amphibolips quercusinanis.

(Oops, I pressed "Submit" instead of "Preview" before finishing, then couldn't edit my text after you replied!)

To follow out my speculation, we would end up with three hosts for Amphibolips quercusinanis:

Northern Red Oak (Q. rubra, from Felt's Q. maxima)
Southern Red Oak (Q. falcata, perhaps what Felt intended by "Spanish Oak"? - this is speculation)
Scarlet Oak (Q. coccinea, no confusion on this one)

To answer your other question, the two currently accepted varieties of Quercus rubra (on the PLANTS database) are:

Q.r. var. rubra (formerly Q. maxima or Q. borealis var. maxima)

Q.r. var. ambigua (formerly Q. borealis or Q.r. var. borealis)

Upon further reflection, it is not necessary to invoke a "third" host. As I determined above (but overlooked in my last note), the term "Spanish Oak" has also been applied to Q. coccinea (already listed as host), so one need not bring in Q. falcata. And as you point out, perhaps "Spanish Oak" and "Red Oak" referred to former varieties of what we now regard as Q. rubra. All things considered, the simplest explanation is that Felt used a common name for Q. rubra in the 1930s that is not widely used today. Ah, the joys of common names and old taxonomy!

I just added an image of a twig. Feel free to frass afterward. Thanks.

The oak in your photo appears to be Pin Oak (Quercus palustris).

Red oak, yes. It grows in a suburban garden. I will try to get more photos and also some galls. I hope that I can catch the gall makers too.