Are you following a pattern, and if so, does it tell you how much yarn you'll need?
If not, the great thing about a baby blanket is that they come in every size from small to quite large.
However, if you want to guesstimate how large it will be, you can do a search on Ravelry (free to join) for your weight of yarn and the amount of yarn you have. The search criteria is on the left side of your screen.
For example, here's a search I did for worsted weight yarn for 900-1200 yards of yarn.
Once you've narrowed your search this way, you can click on individual projects to see exactly how much yarn was used. Find one that's close to your amount and check out the size. Of course, there's nothing scientific about this method, but it should give you a ballpark.