Each foal will vary but I find most of mine have been in a 2'6-2'9 as weanlings.
Your best bet is to measure and get one slightly larger, and the ones that have velcro on the chest are better as they adjust better to fit.
If you plan on buying a dog rug for your foal, just remember that these are measured differently - they measure along the top seam from the top of withers where you want it to sit, to top of tail.