Two inches one way or another isn't going to make any difference. Pick the length you prefer. If you were trying to decide between 12" vs. 36" -- then you might find some differing opinions on which would be a better choice.

Are you asking about a grab bar as you enter the shower/tub area or on the inside wall of the area?
We have a 19" vertical bar as you enter the shower. I do not think a smaller size would make a difference. A longer bar (24" or longer) would be placed diagonally on the inside wall of the area.
Be sure to attach bar to stud for security.

We converted our master bath tub/shower to a shower only. I have MS and getting in and out of the tub was difficult. We found the a 12" vertical grab bar at the entrance was more than sufficient. The length of the bar on the inside would be determined by your preference- do you ave a seat, when is it located, size of area, etc. For us an 18" bar, horizontally placed in the center of the inside wall was more than adequate.(We don't have a built- in bench as it would have been too far from the controls to be convenient. Our seat is movable so it is out of the way at the end of the enclosure when my DH uses the shower and I can move it up when needed. There are several nice alternatives to a built in bench.)
Actually horizontal is the recommended placement for security. Much better leverage from a seated position, also wet hands may slip on a diagonal bar in an emergency situation. Ours is 33" from the floor, or at my wrist level when standing.
Pam

Thanks for the info. I really have never really noticed the placement for them in anyone's bathrooms much less the size. This is all new for me. Don't want a great big obvious one in our small area so was wondering about the 12-14" size.

Had a knee replacement and it is nearly impossible to get in and out of a bathtub.

Like the idea of one as you go into the shower, too. We are getting on up in years, so this will be a great safety precaution.

Dora Lou