You can by all means resize it to however large you want. The original resol. on the picture you provided should at least do a clear 8x10 - I'll drop the Keneds from it and do a different border and then send you file attachments of both via email. That way you can modify it, print it, use it as a signature, etc!
Edit in - I tried doing some resizes for you and I wouldn't go any bigger than a 5x7 unless you want to lose clarity.