Size: enough for the CPU core / heatspreader. Note that you don't have to cover all of the heatspreader, but the more the merrier. You should however endeavour to cover the entire core if it's a non-64 AMD (or old Pentium).
Paper thin: because when you've pressed down hard and then scraped off, you're effectively left with a layer of TIM the thickness of whatever you're using as a mask. I use Post-It notes (the yellow sticky things) but you can also use masking tape or whatever. As regards any residue left by the stuff you're using as a mask - doesn't matter as long as it isn't in between the CPU and the HS, but you can give a little wipe with a alcohol impregnated cloth if you're fussy.
For both AS-5 and G-751, tinting doesn't really work, so don't bother. The object with both TIMs is to minimize how much you work it. If you try and adjust the finish, you just end up mucking it up. Or at least I do...
Oh, and one last thing. Remember that both of these TIMs take ages to 'cure'. They both talk about taking 200 odd hours of heat to fully cure, and CPU temps drop up to 5C in the process. So if your initial results look poor ... fold for a while, switch off, fold a bit more etc. etc. etc.
