My wife has this problem, and we do the whole cranberry thing. There are also homeopathic over the counters that help with it. Sometimes I get them, though rarely. My wife got them a lot when we were first married and doc said it was common with new couples. The peeing right afterwards helps.
Actually, a colleague of mine told me to tell my wife to pee in the shower every morning instead of using the toilet. Seems her doctor had told her that standing up makes you more likely to empty the entire bladder, thereby leaving less chance of infection. Wife tried it for awhile and it seemed to help, but it freaked her out. I saw no problem with it as there is plenty of water from the shower to put it down the drain.
Still, though, wife has vaginitis (or somethin like that) and has some pain during sex depending on a variety of factors. If we've not been together for awhile cause of work or whatever, it's often worse. I never related it to size, and neither did she since I'm the only guys she's been with. However, that could be a point.