A proper analysis would need to know the size of the windows, their U-factor, and the insulation R-ratings, then surface area, things like 8' or other ceiling heights. If nothing else, you can probably see what type and amount of insulation is in the attic. How thick the walls are would give you an indication of the max possible insulation (like are they 2x4, 2x6, or other construction.
The only time a bigger a/c unit is useful is if the whole house is heat soaked, and you want fast cool down. A smaller one will be more comfortable if you turn it on at the beginning of a heat wave (or like I do, at the beginning of the season!), and leave it on. The whole house can absorb a lot of moisture, and it takes awhile to dry it out. Makes it worse if you open the windows at night - it may be cool, but you're adding back lots of moisture into the structure that you have to remove the next time you turn on the a/c unit.
A unit a little small may mean it will run all of the time, but it will also be drying out the air much better than one that turns on and off. In the first case, it may get a little warmer in the house, but it will be dry, and most people will still be comfortable. A cool, damp house is just not comfortable. There is no one right answer - each house is a bit different, and the use pattern may be different. A two-stage compressor can be a help, as it can act like the big one to cool things down fast, then switch to slower speed to maintain it. A variable speed fan on the furnace or air handler can make a big difference, too, as those tend to start out slow, which pulls more moisture out of the air, and at the end of the cycle, ramp down to extract all of the coolness in the coils.
Without a good heat load analysis, most places tend to opt for a bigger one, but that may not be the best choice...they cost more to buy and run, and if not needed, aren't as comfortable.