Assuming your rear hub is a quick release type, not nutted, rear axle diameters are nearly all 10 mm. "Forte" is Performance's house brand so it should be 10 mm.

Some nutted axles were 3/8" or 9.5 mm and, if that's what the frame originally came with, it may be the problem. A few strokes with a file should solve the problem but be sure to file on the lower face of the dropout (further assuming horizontal dropouts) to avoid alignment problems.