Your best bet is to wait for the surf shop sales, they will have loads of different sizes to try on, and advice on what to get.
I use a 3mm all through winter, but then again I have loads of... erm... "Natural insulation" to help keep me warm


If you're skin and bones then perhaps a 5mm would be the go, try to get one with thinner arms so your movement isn't as restricted.
Booties and a helmet make a huge difference as to how cold you get, I was quite surprised

hth