Choose from the following:
A 12" lettuce bowl of mixed lettuces.  Trim as needed, leaving about 1.5" at bottom and don't remove heart.  
 
A 10" hanging basket of baby romaine(either all green or green & red). Can just trim what you need with scissors-leaving about  1.5" at bottom.  
 
Keep in a cooler area during heat of summer to prevent bolting too soon.  If starts to bolt, then they are done(they start to turn bitter).  But all these varieties are more heat resistant & slower to bolt.   Full to part sun-but partial sun in heat of summer will help keep them longer.