Ask Mr. Smarty Plants

The ultimate size that any plant attains depends on a number of factors the most important of which are soil, water, light and temperature. (the genes of your individual plant count too). In College Station you can provide the part shade and sandy to clay dry soil Malpighia glabra (Acerola) prefers. Temperature is going to be a bigger challenge.

Malpighia glabra is a Texas native plant most common in far south Texas, although the USDA shows it to be distributed as far north as Dallas. The Texas A&M Native Plants Database talks about its desirability as shrub for gardens south of Austin, but also recognizes root hardy plants in the Dallas area. Here is a previous Mr. Smarty Plants answer discussing freezing problems in the Austin area.

In the College Station area it is likely this plant will die back to the ground most if not every winter. After the first freeze, the plants will need heavy mulching until new growth emerges in the spring.

The Texas A&M description says Malpighia glabra can attain heights up to nine feet. Since yours will be starting from the ground up almost every year, they almost certainly will never get that tall. They will more likely be about two feet tall with a spread of about two feet. Thus, a spacing of two feet should give them plenty of room, but achieve an appearance of dense growth.