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).