Screws: Which ones go where?

I just replaced my optical drive and I was very careful to make sure not to loose any of the little screws that I removed from the exterior of my MacBook Pro, yet I have nearly 20 screws left of various sizes and dont know which ones go where. Out of the 20 screws there are 3 different sizes... most are very very small, then there are less than 5 that are just small and another group of less than 5 that are 2/3 times the length than all the rest. Ive already reconnected the ribbon cable attached to the logic board and flipped the computer upside down... This is when I realized that, well, Im screwed. When the guides refer to phillips are they referring to one specific size? My main concern is where the longer ones go because i dont want to risk damaging anything.

PS This is my first time attempting to fix my computer myself and everything was going great thanks to this wonderful site I found. The step by step guides are awesome! Ive already recommended it to a few people ;)