Ask each contractor to see their heat load calculations. This should be how they size the system. If any of them are giving you a bid with out running a heat load, then they are just guessing ( using rule of thumb) on the size of the heat pump. This is a big red flag.

An inexperienced contractor might assume that a 5 ton unit will give you 60,000 BTUs/h ( 5 x 12,000 = 60,000 ). This is a common mistake.

After you get all of your bids and determine what the heat loss is of your house, I would then ask each contractor to give you a bid based on the size of heat pump that meets your heating loads.

In other words, give them the list of what you want installed so that all the bids include the same size heat pump, the same type of loop, etc... along with a list of referrals from jobs they have completed.

Then you can pick the best contractor with the best price.

Very seldom the best proposal come the lowest price.

I echo what Geodean says.

For $49 you can download and use Hvac Calc to do your own load calc if you are so inclined.It'll take 2-3 hours to figure out and key in your house, but it is a good cross check against what the contractors come up with. Oversizing is rampant in the HVAC industry, but I like to think Geo installers are a cut above and won't do that. Running your own load calc lets you fiddle with window specs, insulation levels and types, overhangs etc so that you might determine some choices to cut your loads.

Elaborating on geodean's tonnage comment - actual btuh output depends on a variety of factors unique to each installation, the most significant of which are source water temperatures and flows, summer and winter. Major manufacturers have product specs online with tables to determine actual outputs based on conditions.

Some contractors may be reluctant to share load calcs with you since done right they represent a couple hours work and there is a very real risk that unscrupulous customers will take that info and use it to bid out the job to other contractors who then underbid. Doing your own load calc or offering to pay a bit for it (negotiate that payment as a credit toward the system if whoever did the calc gets the project)