Mark, I agree with Brian about the advantages of having a small cockpit, depending on your planned use. I'm building a boat for expedition....I may want to be able to put bulky Items forward of my foot braces and between my legs. I want a bigger hatch opening to allow for that.

Use aside, you need to measure your cockpit as described in the skin-on-frame litterature. See "The Aleutian Kayak" by Wolfgang Brinck (Ragged Mountain Press/McGraw-Hill). He describes a method where the paddler extends his legs into the cockpit, and if his knees clear the coaming and fit in, given a certain knee-leg position, the cockpit is big enough.

If you do decide to alter the size of your cockpit anyway, you may need to trim it bigger than you want because you may need to trim outside of the indentations used to secure spray skirt...some of those cockpit coamings are recessed, and you may need to trim outside of that as well...you might end up needing to make yourself a new sprayskirt, because after all is said and done, you'll have made your cockpit bigger than the standard sprayskirt will allow. Good Luck!!!!
Bill
PS
You could also sell the boat you have now to buy materials for a wooden boat.

: What kind of boat is it? Getting in and out of small cockpits takes some
: getting used to, but it's not difficult once you get the hang of it. I've
: got a 36" inseam and all of my boats have small cockpits, by choice.
: I find I have better control and never have to worry about a leg popping
: of a thigh brace or falling out of the cockpit when inverted.