Can I ask what the best unit sizes are to work with in a 3D modelling package like 3dsmax is? Does it really matter?

Yeah, this doesn't belong in .Net

Thank Washu :) Was just trying to figure out how to move it here :)

It probably doesn't matter as long as you are consistant. If this is for use in a game then you'll want your units to match the scale used in the game's world space. That decision should probably be driven by the type of game you are making, and the scale on which most of the objects are made. If most objects are on a human scale then 1 unit = 1 cm or 1 unit = 1 meter is usually good. For an RTS 1 cm would probably be too small.

In 3ds max specifically, the camera (especially when zooming) tends to behave differently at various scales. Making your scale too small can also lead to the near clipping plane cutting off objects near the screen. If you're free to decide your own scale, specific issues like those can have some weight on the scale you want to set for yourself.

You can always scale your stuff after loading them anyways. Work in whatever is comfortable, and then scale as you load.