I just got a Dell U2713hm to hook up to my Mac Book Pro. I thought it was a 2560x1440 monitor but I'm only getting 2048x1152.
I'm using the mini display port to VGA, am I just stuck with the lower resolution?
It's probably a limitation of your adapter. Many cards have a 400MHz DAC for the VGA output which only gets up to 2048x1536 or so. Try a dual-link DVI cable? (Single-link DVI or HDMI won't get much above 1920x1080 so you will need a dual-link cable.)
Edit: it you're using Apple's official adapter, it should go up to 2048x1536. That will still only get you about 3 million pixels per frame. If you want 2560x1440 you need something that can handle 3.6 million pixels.
The problem is definitely the mini display port to VGA adapter.
I have achieved 2560x1440 with the Dell U2711 and a Macbook Pro by using a DisplayPort to DisplayPort-Mini cable.