This is a rough guide, best idea is to look at the sat footprints on lyngsat or simillar
42 east - 90cm
39 east - 90cm
28 east - 60cm
23 east - 90cm
19.2 east - 60cm
16 East - new satellite Eutelsat W3B due to launch 15th September 2010 giving 70cm receoption in the UK & Central Europe
(edit replacement satellite failed size required 90 cm )
13 east - 60-80cm
9 east - 60cm
7 east - 80cm
4.8 east - 80cm
1 west 80-100cm
5 west - 70cm
8 west - 80cm
12.5 west - 80cm
15 west - 80cm
24.5 west - 80cm
30 west - 80-100cm
Thanks to origional poster Ten Below