Hit CTRL + A to select everything in the directory
Go to File > Properties
Wait until the Size: field finishes counting up all your files and stops changing. This is how much space you're currently using (the max is 1 GB)
Alternately, you could mount your userspace as a network drive which treats it just like another hard drive. This makes it very convenient to add and remove files as well as check how much space you have left.
Hit Apple + A to select everything in the directory
Go to File > Get Info
A window that appears similar to the following will appear. It may take a few moments for your computer to finish calculating the disk space (the max is 1 GB)