Thanks for the link. I'll have to weigh my defrosted mice to get an idea as to what they are and how much I am feeding Sid. 
The shop where I get mine from call them :- 
Pinkies, Fuzzies, Small mouse, medium mouse and large mouse.
** EDIT **
Here are the UK sizes as I understand them to be:-
1 Pinkie    (2-3g)   every 5-6 days (Snake = 4-15g)
2 Pinkies   (3-4g)   every 5-6 days (Snake = 16-23g)
Small Fuzzy (5-7g)   every 6-7 days (Snake = 24-30g)
Fuzzy       (7-9g)   every 6-7 days (Snake = 30-50g)
Fluff       (9-12g)  every 6-7 days (Snake = 51-90g)
Small Adult (14-20g) every 7   days (Snake = 91-170g)
Adult       (20-30g) every 7-x days (Snake = 170+)
Also, I am lead to believe that this was created by a guy called Roy Munson, is this correct??
** END OF EDIT **
Going by the linked guide Sid should be eating weaned, which I'm presuming are medium sized mice as the next and last size up on the chart are adult.