The leach field size is heavily dependent on the perc rate/soil type, as well as nearby streams, ponds, rivers, etc. The size of the field is usually determined similar to this:
Amount of wastewater capacity needed is determined by a set number of gallons/day per bedroom - the number will probably vary depending on where you live
The perc test determines how much wastewater your soil can absorb and thus how many linear feet of leach line are required per gallon/day. There are some leach pipes called "infiltrators" that are supposedly better than the standard round-perforated (ie. more capacity in a shorter length), but it'd be up to the local official as to if/how these can be used.
The tank size may need to be upgraded, depending on how large the current one is and on your local codes.
Many areas now are requiring sand-bed or raised-bed systems be installed when a new or upgraded system is installed. This can be VERY expensive in some cases and will take up a fair portion of your property with a large mound of sand.
All in all, Bob has a good idea about calling some contractors and getting ideas and even some quotes.
Adding bedrooms... either you need them or you don't. If you need the extra room and it's not just going to be a convenient spare bedroom, there isn't much you can do other than make the kids share bedrooms (like the bunkbed suggestion).