When can I judge my beagle mix's adult size?

I think you'd be pretty safe to buy a standard 36" crate. But yes it would help to know Beagle mixed with what, to be more sure - much as, as ever, mix breeding can produce huge differences in the fully mature size, in each puppy. Eg. my sister adopted a mix Jackie/Springer from the RSPCA and she has legs. I came across a litter brother in town a few years back (amazing coincidence although we knew the town she came from was where I live) and he's far more like the Jackie, same big body, but no legs!!

You may have to wait on the eventual collar/harness size (why a harness - these only really encourage a dog to pull - they are used on pulling dogs, to pull!) however. I've had to by 3 collars for my Basset already, and even so, he's now on the final hole of his 18" one, which I'd hoped would be his last!!

ps You may be better to buy a head-collar for your Beagle x.

Beagles will generally reach full height between 8 and 9 months old, but will continue to "fill out" some. Depends on what the mix is to know for sure. Since she is already 18" tall (assuming that is at the shoulder), she is probably mixed with a larger breed, so she may continue to grow some after that.

I ussually just get the adjustable collars when I have a puppy.. then upgrade to a better collar later when the dog reaches its adult size.

I think a fair weight guess will be between 30-40 pounds. There's not really any accurate way of guessing adult size tho, expecially in a mixed breed.

It would help if you mentioned the breed she is mixed with. Some of the larger dogs such as shepherds do not reach their mature size until they reach 15 - 18 months.