I would like to mount a trolling motor on the bow of my V-Hull. A buddy is going to make me an aluminum plate 1/8 inche thick, but we don't know what size to make it. How much space is needed ?

Big enough to accept the mount on your particular motor

I do not know if this will help you here is a picture of my boat.

A much bigger picture

http://home.comcast.net/%7Eteam_dougherty/boat00.jpg

an almost view from the top

http://home.comcast.net/%7Eteam_dougherty/boat5.jpg

I haven't picked one out yet. Like to find one with the smallest base.

Thanks for the pictures.

Use cardboard and make a template.  That is the easiest way to do it.  Here is mine.

Its not a taken at a very good angle, but here is my boat