Won't fit as in too big...or too small.
The tranny line is 3/8". The "seal" part of the connector has 2 o-rings and a spacer. I slide on one o-ring, then the spacer, then the other o-ring on the male side. Then push the female side on to the nipple until it "clicks". With the emphasis on click.
If you try to stuff the o-ring, spacer, and o-ring in to the female side it'll likely get crossed up. If you don't have the spacer, it'll be loose (as in, won't seal). If the clip that holds it all in is too small (they come in 3/8" and 5/16 [for fuel line] ) it won't fit tight either.