10x8 ft is quite tiny if your claim of insulated well is formal and the ceiling is 8ft.... you can use a 4000 btu roll around unit....
Now in reality to size an AC unit you perform a heat loss... Here is an example... I have a 10x8x8 ft area that is occupied by 3 people and is being used as an office space where 3 PCs and a printer will be running and has 2 windows that have a loss of 400BTUs. The expectation is to hold this office at 72F even if the ambient temp is 100+F ...the building has a R19 insulation.
This example is average...and 3 people have about 800BTU loss each...PCs and Printer equal about 700BTUs and the building is at about a 2000BTU need.... So we add them up...2400+2000+700+800= Just under 6000BTUs to perform the job if everything is 100% efficient... we have only 30 % efficiency in refrigeration systems. So we 6000x.7= 4200 and you get about 10,000 BTU unit for this case.
So in your case and my case.... 4000BTU to 10K Btu will work fine...depending on what load you are looking to cool. if the room is really leaky (not well sealed) Your AC unit might really whip out the drainage.
I cannot properly cool a garage in my house.... too many poorly builtin factors...uninsulated garage door....almost no insulation in walls and ceiling and pet doors installed.
1994 Fleetwood 33H
1995 Fleetwood 30H (parts vehicle)