How can I increase the water pressure in my house (well)?

It is unlikely that your pressure tank is undersized. The pressure tank does not effect the pressure in the water lines in your house, it only tells the well pump to turn on to send water to the house You have a blockage some where or one of the components is installed incorrectly. Try doing some testing first. At the base of your pressure tank there should be a boiler drain. It is just a spigot, open that up and see what kind of pressure you have there. If you do not have good pressure there shut off the electric to the well and remove the line from the well to the pressure tank from the pressure tank. There should be a check valve right there so remove it also. Turn the electric back on and see what pressure you have coming into the house at that point. If you have good pressure coming from the direct line from the well. Your check valve was either put on backwards or it is not working properly and you need a new one. Be careful you could get water all over the place as you do this. If you do not get good pressure at that point the line from the well to the house is broken or plugged. If you have good pressure at the boiler drain at the base of the pressure tank you have some kind of a blockage at the distribution manifold for the pex tubing or there is a blockage between the pressure tank and the manifold. There is also the possibility that the plumbers didn't clean the pex tubing properly before they made various connections and there are small particles of plastic pex plugging something up. You have to find out what the blockage is, and adding an additional pump inside the house won't do a thing.