Cost difference on one specific one:
0402: £0.011
0603: £0.007
0805: £0.024
Depending on the actual resistor needed, I may or may not be able to get the size I want to use and will have to use a different one. For example, sometimes I can't get a 10K in 0402 size and have to use 0603. And I'm trying to stick with Farnell since you said you're in the UK.