Hi Arran, first you need to decide what your final render resolution is going to be because
a) You can determine what size the texture is going to appear in the final image.
b)What level of detail you need which depends on how close to camera it will be seen
An example might be of a character and lets say that your final shot resolution will be for TV (i believe you are in the UK) so it will be Pal 720x 576 so at 1:1 if you are doing a full screen shot of the char then he will be seen at 576 pixels high so if you don't intend to zoom in then a texure of 512 x 512 might do and definately 1024 x1024.
If you zoom in you will go beyond 1:1 and so the texture will blur . To stop this from happening you will have to increase the texture resolution so lets say you will zoom in to a close up of his arm and hands, if they fill the screen then the arm and hand texture alone would have to be at least 576 pixels and if your whole body is one map then it might have to be 2K just so that the arm and hands are at least 512 when zoomed in.The other option is to break up the texture maps so you have more than one for the character and only have larger textures where you need it, so you could have 512 x 512 on the legs and body and maybe 1024 x 1024 on the arms and hands and perhaps the head if you intend to zoom in on that.
Some people use a rule that the texture should be twice the final resolution but really you can optimize this a lot if things are just in the background and will never be seen up close.
Hope this helps a little
Heres a link that might explain it a little to you
2 x Modeling Challenge Winner