Ask Mr. Smarty Plants

"S. buckleyi and the the similar Q. shumardii (occurring mainly from the w edge of the Blackland Prairie e to e TX) hybridize along a narrow zone of overlap from the Cooke and Grayson co. area near the Red River s to the vicinity of San Antonio (Bexar Co.). To the w of this hybrid zone "pure" individuals of Q. buckleyii can be found while to the e "pure" Q. shumardii occurs."

So, the tree you have in your front yard may be Q. shumardii, but it also might be Q. buckleyi—or a hybrid of the two. You are correct that Q. shumardii (maximum ~100 feet) tends to be taller at maturity than Q. buckleyi (maximum ~50 feet), but we don't have any figures for hybrid maximum heights. Our information for the width, or spread, of the trees is that Buckley oak is up to 60 feet and that is also the maximum spread for Shumard oak. Since the width of the tree generally increases with the height of the tree, the maximum spread of the tree shouldn't occur until it is well above your roof line. So, unless you are worried about the canopy of your tree spreading over your roof or encroaching on your neighbor's yard, it shouldn't be a real problem for your small yard.

There is one other red oak, Quercus texana (Texas red oak), that is native to Texas. It occurs in extreme East Texas in different type of soil and climate and wouldn't be well-adapted for Austin.

You can read descriptions, see illustrations and distributions of these 3 Texas red oaks—Quercus buckleyi, Quercus shumardii, and Quercus texana—in eFloras.com, the online version of the Flora of North America.