You should aim for an intake of at least 1.5L of fluid a day, but preferably over 2L if you can manage it - doesn't necessarily have to be plain ole water, but can also come from soups, jelly etc.

As Bath said, there is on correlation between body size, and fluid requirements, and there is not really any correlation between your size, and the capacity of your bladder. For example, how many short women out there with large breasts, and tall women with small breasts - same thing for men and their "appendiges"!

Anyway, the "average" bladder holds about 300ml of urine - about the size of a softball.

You asked if there is any way you can train yor bladder to hold for longer...the only way that you can do this (without surgical intervention!) is to do your pelvic floor muscles, and do them properly. Call your hossi, and ask to speak to a physio about them if you can, to check that you are doing them correctly. Some hospitals also have Continence Advisers who specialize in, surprisingly, continence, and would aslo be able to help you with the correct technique for your pelvic floor exercises, if they are able to talk to you at all (I know ours is always really busy, so that is why I suggested a physio first).

If your issues with "holding on" continue after you have given birth, and it starts to worry you, you could ask your GP for a referral to a urologist, who could help you by addressing any issues you have with your bladder, and your frequent UTIs (again as Bath said, you may have a bodgy valve, or sphincter somewhere in your urinary tract that is contributing to your UTIs)

Good luck, and remember your pelvic floor muscles!