You mustn't oversize an air conditioner. Air conditioners both cool and dehumidify the air. If you install an oversized AC, it won't run long enough to dehumidify the air before the room is cooled down, making the air feel clammy. Reducing the fan speed does not slow the compressor whose job it is to cool and dehumidify the room. Also, if an oversize AC will cycle frequently which may be annoying to live with and reduce the life of the unit. Finally, a higher-BTU model may require a circuit update. For example, the 8000 BTU LG LT0816CER runs on 110 volts while the 10000 BTU LG LT1036CER runs on 220 volts.*

-- non-expert

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*Just check the voltage requirements before buying

I live in the desert, so dry and summer hits 120. The room is 230 Sq. Ft. with 11Ft. ceiling and poor insulation. Will 8000 BTU cooling be enough?

Arid climate reduces or eliminates the concern of increased humidity via oversizing. 11-ft ceiling adds more air volume in the space to be cooled, and of course high ambient temperature and poor insulation increases the cooling load.

These BTU Calculators should be considered a very rough estimate but have a variable for ceiling height and one has a variable for desired temperature change. Play with the numbers to see the effect. (The BTU sizes it calculates for the details you stated come up larger than I expected. Consider your past experience with whatever size unit you now have.)