Does house size matter when it comes to water pressure? I have a 3000sq.ft. 4 level split and the Master suite is on the top floor. I use a well and live in the country. Pump is 3/4 HP jet pump. Current Pressure gauge at tank is 50 psi. Heaven forbid if someone flushes a toilet while one of the showers is in use. Flow never good to begin with but if someone turned on the washer or flushed the flow is so low that if goes back to the wall and just trickles down.
First, the square footage of the house doesn't mean a thing in this case. The important factors are water pressure and supply line size and the height of the fixtures above the pressure tank and the pressure tank size. The well pump fills the tank, then the air bladder applies pressure to the water in the tank. a check valve stops water from going back towards the well. When you call for water, the internal tank pressure pushes it through the pipes.
A few different problems may exist here that would cause your problem.
The tank pressure membrane may be waterlogged. It will still show static pressure, but drops to a very low PSI when a faucet is turned on. Often, you will hear the pump cycle more that usual in this situation or you may see the water pulsate instead of a steady flow. What is your static vs open line pressure?
Another common problem is the backflow check valve is leaking. This causes water to be pushed back towards the well. the tank pressure will slowly decrease between pump runs.
If this problem has been long standing, then the pipe size going to the second level may be too small. Second floor fixtures or ones a longer distance away should be fed with at least 3/4 inch line, not 1/2 inch line.
The last factor may be that the well is not delivering enough gallons per minute. If the pump has to run for excessive time to fill the pressure tank, this could be the problem.
I'm sure there may be a few other items to check, but these are the most common problems.
A more common problem would be a clogged filter. Also a jet pump is for a shallow well. 25 feet or less maybe yours is more. Has this always been a problem? How long have you lived there? Use an outside spigot and test how many gallons per min your pump is. Let the water run for a couple mins until your hear the pump kick in. A 3/4 pump is rated around 5 gallons a min at 25' well depth. If you cant fill a 5 gallon bucket in a 90sec something is wrong with pump or there is a blockage between spigot and pump.
Poor plumbing layout (pipes too small) makes a lot more difference than square footage on the "heaven forbid that someone flushes a toilet" front.
Depending on water chemistry you could also have a mineral build-up in the pipes, partially blocking them (making them act like much smaller pipes, and exacerbating this sort of problem). But there are a LOT of houses where far too many fixtures (or the whole house) are being run through a 1/2" pipe.
@Shirlock Homes gives a good list to look at.
In contemplating fixes, you might want to consider replumbing with PEX, in the "standard for PEX" arrangement where every fixture gets its own line from a manifold near the pressure tank; thus, they all see the same pressure, and one fixture does not significantly affect any others (if the pump is keeping up) - so you do need to check on the pump performance, near the pressure tank.
Observe tank pressure, have someone else run around and flush all the toilets, while you observe tank pressure, report back; or use a drain fitting (there should be one near the tank) and a hose or bucket to draw water right at the tank. A typical functioning system will slowly (should take at least 5-8 gallons of draw-off) reduce from 50 to 30 (or 40 to 60) and then the pump will kick on and raise the pressure back up fairly quickly. A waterlogged or failed membrane system will drop almost instantly from 50-30 (or 60-40) and then the pump may cycle in many short bursts (bad for the pump - one reason a functioning pressure tank is needed) up to the shutoff, off, down to the cut in, on, etc.
A pump (or well) that just can't keep up will very slowly raise the tank pressure.
Try it, observe, take notes, and report back.
One of the more definitive tests is (with the rest of the house not drawing any water, or shut off if there's a valve handy) to drain just until the pump kicks on, stop draining immediately and start a stop watch. Stop the watch when the pump kicks off. Drain into a bucket (or several buckets) just until the pump kicks on again, and measure the amount of water (you can also repeat the time measurement here). From this you can assess the health of the pressure tank (need its size, also) & the output of the well pump.
Another test is to use a hose and a bucket (and stopwatch) on the drain and try to adjust the flow rate (when the pump comes on) so that the well pump is running continuously at ~40 PSI (on a typical 30 PSI on/50 PSI off setup) both to asses the flow rate, and to see how long it can maintain that flowrate before you have to close the valve more to maintain pressure. That will tell you something about the pump and the well.