There is no actual bow that manifests just an arrow that gets shot at a target. you can actualy wield things in your off hand there for the size of the bow doesn't matter.
i do happen to be perfectly aware of that point, however...
size does matter, size always matters. in this case, if one reads the fine print, it says that the arrow is manifested as if shot from a long bow. if one was able to use the shape mind blade ability to change the size of the virtual bow, then ones damage should increase or decrease. alternately, if it only changed the size of the arrow (perhaps a more valid interpretation if one was even inclined to allow it) it may not make much of any difference, except visual... which might be made to matter if one is clever.
darn it... now i'll have to reread the lucky mod too. i think my dm would start throwing things at me if i tried it, but oh well. =D