My standard shetland is now in a 4'9 now hes fully grown, he even has a show rug thats a 5' !
Also a lot depends on the fit of them, we have had a few where the surcingles havent been able to go long enough! or we have neck issues!
'Normal' rugs don't generally fit shetlands, either too long, as above, or not long enough and the belly sticks out, as below! There are companies who make special rugs for shetlands. http://www.equineclothing.co.uk/ for example
The 32" is a miniature horse ( finer build ) and takes 3'6'' and the 31" is a miniature shetland and takes 3'9'' rugs.
All i would say is if you measure dont just get normal rugs as stated above as most are too big around the shoulder.
These are a very good company too i got most of my NZ rugs from them Dinky rugs
Also there is a lady called Lynn from pampered ponies ( site on ebay ) i have her number if you need it will gladly send via pm. She does excellent rugs and very cheap compared to most and excellent quality.
my 3yr old filly wears a 4ft3 or 4ft6. she has a derby house pony rug that fits her perfectly! is plenty deep enough to cover her belly, also have a masta and shires where her belly sticks out like the pic above.
My standard Sheltie is a 4ft9, but I think she's quite a tall standard if you know what I mean. I use a Rhinegold (normally I hate this make) turnout which fits her well. I have an Olympian stable rug for her but it is a bit wrong proportion wise.