My sheltie Isla was quite small at 10 weeks when I had her.  She had been born the biggest gal of the litter (4 bitches) but by the time I bought her she was the smallest.  I met her mum who was quite small.  However lately she has put on a huge growth spurt and now measures 14 ins at the shoulder at 6and a half months.  The ideal height for a sheltie bitch is 14 ins - especially if you want to show.  I am pleased she has reached the correct height but worried she might get bigger.  Anyone any ideas if this is likely or is in the lap of the gods??  So from a little squirt she has become quite an eyeful as her adult coat is coming through and she looks a proper hairy mary.   Any sheltie owners out there? - I guess some breeds grow at different rates?

Snowflake

Snowflake,check this out-
http://bowlingsite.mcf.com/size/size.html

I'm no Sheltie expert and you are correct different breeds and, in fact, different individuals within a breed grow at different rates. But I would think your lady has all but finished growing in height although she may get a bit longer as the small bones grow last and the spine is made up of small bones.

In my breed where I want a bitch to be 19 1/2 inches at 6 months I would expect her to be 18 inches and grow in height to at least 9 months if not a year.

I would think it very likely your pup will go oversize, which seems more common than undersized in this breed.