Know processor speed of our application
What I did for my project (for from unfinished):

For memory usage:


Count for each item (global variables):


How many you need to store in SRAM
How many bytes each type cost

Add some extra for stack, and temporary variables
Check what you want to optimize


For processor speed, it's a bit trickier. You should decide what is the most performance and critical part of your program, than decide how much instructions you need to perform it against the speed you need. You could analyze this by writing the critical part and check in the assembled code the exact amount of instructions, calculate it by the clock cycle used for that instruction and sum all up.