3.3lb can is 1.5-1.7kg,the weight of the average can of Cooper's,Munton's,etc. They generally make 23L,or 6.072USG. At 5G,the beer is a little stronger,but not by much. But 5G with one is fine.
If it's pre-hopped do not boil it. That messes up the hop profile. Not to mention,mailard reactions. Iuse plain DME in the boil for hop additions. Typically,1/2 of a 3lb bag in a 2.5-3 gallon boil. I add the remaining DME & all LME at flame out.
So you could get 2-5 gallon batches out of the 2 cans done my way. You can look at my recipes for more details on how I do it.