I'm using a MBP 13" and want to connect it to an external monitor. What is the largest resolution supported for an external monitor?
From what I know this MBP is using VGA intel.
MBP 13" early 2011
The tech specs for your model are on Apple's support site.
If you have a display that has a native Displayport input (many mid-range and up displays from the past 2-3 years), you can output up to 2560x1600. Typically these displays have a full-size Displayport connector, so you'll need an adapter or cable that goes from Mini Displayport to standard Displayport - they can be found pretty cheaply at Monoprice or Amazon.
You can also output up to 2560x1600 using a Mini Displayport to Dual-link DVI adapter, however these have been problematic in the past and run around $100+. Not a great option unless you already have a monitor with a resolution higher than 1920x1200 and it doesn't have a Displayport input.
Using adapters for single-link DVI, HDMI or VGA, you can support up to 1920x1200.