It's not the penis size that causes it.. Here are suggestions I found for females that experience this after intercourse....

• Changing sexual positions so that a penis or finger is not rubbing or "knocking boots" directly against your urethral opening (pee hole). Sexual intercourse and/or penetration can push bacteria into the bladder.
• Emptying your bladder before and after sex could help flush out bacteria, which can reduce your risk.
• Learning about alternatives to spermicides, including condoms coated with non- spermicidal lube, other contraceptive options, and safer sex techniques, and discussing them with your partner and health care provider, are good ideas. Spermicides can alter the variety of bacterial flora in the vagina, killing off friendly bacteria and allowing pathogenic ones to flourish, causing an infection.

Hope this helps so you can get back to work! :)