What can a couple do to reduce pain during sexual intercourse caused by mismatched genital size (i.e. he is too thick to fit comfortably inside)?

Dyspareunea: A couple things. First, lots of foreplay to help lubricate the vagina and make the vagina more capable to stretching some. A good liquid water based lubricant may help. Also using a sex toy that is nearly your partners size may be helpful as well, so you can insert it at your own comfort level. Lastly, make sure your partner understand he doesn't need to insert deeply to have fun.