For EDGE dimensionig, you should actually start from the Downlink RLC throughput required by users, on Air interface + the amount of EDGE users. That is your initial assumption. For example, as a EDGE user, I would want to get at least 130kb/s most of the time.
And in average, you can assume there are 1.5 users at the same time in one cell. So you need : 6 or 7 PDCH with a throughput in MCS9, etc etc
From this, you will deduce:
1/ number of required TS & TRX
2/ capacity required on Abis
3/ capacity required in PCU (MFS in ALU)
4/ capacity required in Ater PS & Gb interface