Hmm, It should be possible for some larger than average dwarves to singlegrasp a greataxe.
[BODY_SIZE:5:0:70000] (average size for masterwork dwarf)
[BODY_APPEARANCE_MODIFIER:HEIGHT:75:95:98:100:102:105:125]
[BODY_APPEARANCE_MODIFIER:BROADNESS:75:95:98:100:102:105:125]
70000*1.25*1.25 = 109,375 (max size for MDF dwarf, I think: easily big enough!)
and the needed size is
[TWO_HANDED:77500]
However those guys are quite rare, if you step down one bodysize interval
70000*1.05*1.05 = 77175
not quite big enough
I think Therapist, unless something has changed in recent versions, looks at the average rather than the individual's size on the tab where it tells you what weapons can be singlegrasped.