External display for MacBook

I connected an additional display to my MacBook Pro 13" (mid 2010) and stumbled on a problem. The native resolution of the display is 1920x1200 but after connecting with the MacBook I get 1600x1000 max. On the specifications, it says that the model supports up to 2560x1600 resolution for external display. I am now buying a new display and I wonder what I am suppose to look for to avoid this problem. I wonder if the proportion of the display has any importance. I will be very grateful if someone can give me any information.