Here are the maximum specifications for glove size from the SSUSA (Senior Softball USA) Rule Book. That's the only rulebook to which I have access. I believe the highlighted measurement is the one manufacturers publicize as glove size.
3.6 • GLOVES
Gloves may be worn by any player, but mitts may be used only by the catcher and first baseman. No top lacing, webbing or other device between the thumb and body of the glove or mitt worn by a first baseman or catcher, or a glove worn by any fielder; shall be more than 5" (12.70 cm) in length. Here are the maximum length specifications: palm width (top) 8" (20.3 cm), palm width (bottom) 81⁄2” (21.6 cm), top opening of web 5" (12.7 cm), bottom opening of web 41⁄2” (11.5 cm), 1st finger crotch seam 71⁄2” (19.05 cm), thumb crotch seam 71⁄2” (19.05 cm), crotch seam 161⁄2” (41.91 cm), thumb top to bottom edge 91⁄4” (23.5 cm), 1st finger top to bottom edge 14" (35.56 cm), 2nd finger top to bottom edge 131⁄4” (33.66 cm), 3rd finger top to bottom edge 121⁄4” (31.12 cm), 4th finger top to bottom edge 11" (27.94 cm).