To measure the combustion chamber size:
do this ONLY if you feel confident enough and have a few tools.
Remove the head (only if you have a torque wrench to put her back on!)
Leave the plug in.
Lay the head as flat as you can upside down, using a syringe (10cc/ml) drip water into the head 1cc at a time until you just before you form a meniscus.
or fill the head till you get a meniscus, remove surface tension so water sits flat in relation to the edge of the head and draw it off using the above mentioned syringe.
Similar to this:
http://www.youtube.com/watch?v=QrfEo-Od8yE
Both of these will give you a combustion chamber size. (fairly accurately)
To measure PSI:
If you want to know how many PSI you are generating you need an engine compression tester which sits in the plug socket, give it a crank and it will tell you compression in PSI.
http://www.mityvac.com/hq_images/05520.jpg (something like that)
If you don't have one of these perhaps somebody with one and a slant head would be kind enough to do this for you, if anybody offers make sure your engines are modified in the same manner (or not modified as the case may be) as some mods will create higher PSI readings.
I am sure the is some nasty maths way to figure out PSI from dimensions, I don't know it though.