Contractors are often fearful of installing an inadequately sized unit; they have to replace the whole thing if they are wrong. An oversized unit will provide heat, but the unit goes on and off too often, because the burner usually only has one input - full on and full off - thus producing inefficiency and discomfort. So safety is in the oversize for them.
You can check their estimates against each other and even check one or two room calculations yourself by going to http://www.aresinc.com to use an online Flash4 heat loss calculation program. You can use the outside dimensions of the house as one room and get the furnace size estimate.
Ask to look at a copy of their heat estimate. Most contractors submit room dimensions and specs to suppliers to let them do the calculations. (I teach them how to do that.) A contractor that is not going to do a formal estimate at all is just guessing. You might want to talk to the brighter ones who actually do them.