I'm fairly new to knitting/crochet. I've got a pattern that requires blocking but how much should I expect my item to shrink by? I'm finding making it quite tricky as I don't know what size it should measure before I finish it. Help!
The way to determine how your finished item will look and behave is to make one or more gauge/tension samples.
A gauge sample (or swatch) is a smallish square of fabric produced using the exact yarn, needles and stitch pattern that you intend to use for your actual project. Once you have knitted or crochet your sample, do to it all the things that you are going to do to your finished item: scrunch it up, stretch it, wash it. Afterward, pin it and let it dry. Keep in mind the exact treatment that your article will be receiving. Whatever results occur in your sample is what will happen to your finished project when subjected to the real world.
Many knitters and crocheters make more than one sample, in order to test various treatments to the fabric (eg, hand wash vs machine, one blocking method vs another, etc), and although it requires more effort (and yarn), it will really give you the best answers as to how to treat your future item after you have put all that hard work into it.
To test how much shrinkage will occur when you block your article, make a square-ish sample of about 30cm (12") and block in your desired method. Once blocking is complete, measure the sample again, and you can calculate a percentage for shrinkage, which can then be used on any size finished item. If you have the time and the patience, you can make a few samples, block them in different ways, and compare which method causes the most or the least shrinkage.
I've given you quite a long answer in an effort to be as thorough and descriptive as possible - I hope that this helps.
Blocking usually makes things a bit bigger.
Felting, would shrink things.