What Size Of Blank

So i'm thinking of buying a blank just to have a shot at designing a shortboard, i'm guessing that being in the uk then homeblown is the best place to get my (polyutherane) blank from.
My question is, say i'm wanting to try and design a shortboard with specs... Length 6'4", width 19.25", thickness 2.25". What size of blank should i be looking at buying as a minimum, taking into consideration i aint ever tried this before. Is there a general rule to how much you should oversize your blank in relation to your finished product?
Would a 6'5" blank be too short, should i go for 6'7"?
My question is, say i'm wanting to try and design a shortboard with specs... Length 6'4", width 19.25", thickness 2.25". What size of blank should i be looking at buying as a minimum, taking into consideration i aint ever tried this before. Is there a general rule to how much you should oversize your blank in relation to your finished product?
Would a 6'5" blank be too short, should i go for 6'7"?