How many gb is my iphone 4s?

I just got a iphone 4s from a good friend and the iphone doesn't have a sim card in it so it won't let me get to the home screen (so please don't tell me to go to the settings app).It doesn't say it on the back of the iphone. I'm trying to sell it on ebay, but i need to know how many gigabytes it is. I already asked my friend and doesn't know anything at all about iphones, so he didn't know how many gb. Please some one help me!

Added (1). Oh ya i do know the imei number if that helps

Added (2). When i plug it into itunes, it tells me i need to insert a sim card

Nope the IMEI wouldn't help. Best suggestion would be to get a SIM into the phone & check it that way. Plugging into a computer running iTunes may help as well.

Look at the back of the phone, it'll be written there (in very small text).

P.S. When selling it on eBay you should also say in the ad what carrier it was made for - i.e.AT&T or Verizon or Sprint, since you can't use an AT&T phone on Verizon etc. You also should say in the ad whether the phone has a clean ESN or not if it's Verizon/Sprint (most people won't buy a phone with a bad ESN since it can't be activated).

Connect it to your PC or Mac. Open iTunes and there you will see the storage capacity.