What size air conditioner for a ~230 square foot room with 10 foot ceilings?
The EPA has room size to BTU chart with some caveats for certain situations:

https://www.energystar.gov/index.cfm?c=roomac.pr_properly_sized


  Room size            A/C size
  ---------------     ------------
  100 up to   150 -&gt;  5,000 BTU/hr
  150 up to   250 -&gt;  6,000 BTU/hr
  250 up to   300 -&gt;  7,000 BTU/hr
  300 up to   350 -&gt;  8,000 BTU/hr
  350 up to   400 -&gt;  9,000 BTU/hr
  400 up to   450 -&gt; 10,000 BTU/hr
  450 up to   550 -&gt; 12,000 BTU/hr
  550 up to   700 -&gt; 14,000 BTU/hr
  700 up to 1,000 -&gt; 18,000 BTU/hr
1,000 up to 1,200 -&gt; 21,000 BTU/hr
1,200 up to 1,400 -&gt; 23,000 BTU/hr
1,400 up to 1,500 -&gt; 24,000 BTU/hr
1,500 up to 2,000 -&gt; 30,000 BTU/hr
2,000 up to 2,500 -&gt; 34,000 BTU/hr

  
  Make any adjustments for the following circumstances:
  
  
  If the room is heavily shaded, reduce capacity by 10 percent. 
  If the room is very sunny, increase capacity by 10 percent. 
  If more than two people regularly occupy the room, add 600 BTUs for each additional person. 
  If the unit is used in a kitchen, increase capacity by 4,000    BTUs. 
  Consider where you install the unit. If you are mounting an air    conditioner near the corner of a room, look for a unit that can send
  the airflow in the right direction.
  


If you fall into any of those special situations, that may help you with sizing. Otherwise, since you're between sizes of available units, unless your climate is particularly mild, I'd go up a size to allow a bit extra cooling capacity for hot days.