Hi Denyce The size of a central A/C unit is less important than how the air in the house is circulated.
From your post, it sounds as though you have a central air unit already inplace, that should be part of your central forced air furnace. If so, you should also have 'cold air' (or in the case of A/C, warm air) return registers that return the air to the central unit to be heated or cooled for redistribution in your house. My guess is that the A/C people are telling you that the existing cold air returns are near the floor and are therefore not recirculating the warmest air up near the ceiling, and that retrofitting 'warm air' returns up near the ceiling is what you really need for the A/C unit to cool the second floor. If this is indeed the case, then they are right. Simply installing a larger A/C unit will mean that your lower level(s) will cool very quickly and will get quite cold while the unit is trying to cool the second floor...and this will be an expensive way to do it through much higher A/C electric bills.