A friend has requested a cat shaped cake for her son's birthday. It needs to be enough for 20 people. How do I know it will be big enough? I've only just started making shaped cakes, so I don't know how to judge...??

Figure out the # of servings the cakes are that you are starting with, then guesstimate the % of cake you will cut away (I like to use a graph paper template for this). Then add 10-20% to the amount you think you should start with.

Example:
4 8" rounds = 40 servings (party size)
I expect to cut away 25%, leaving me with 30 servings, should still leave me with at least 20 even if I cut more than I expected.

It really is a guessing game when it comes to 3D cakes, also account for the fact that when the cake is served, you won't be able to cut exact, neat rectangular pieces. HTH.