If you go bird watching, I assume you'll be spending good parts of the day looking through your binoculars. The quality of the lens you look through will determine whether you come home having enjoyed your day, or with a splitting headache.
On the high end, you have the Leitz and Zeiss German made binoculars. Super sharp, and super pricey. On the low end you have brands like Tasco. Bushnell makes good ones for the money, and if you want to spend a bit more, you can go with Nikon, Pentax or Minolta.
The larger models, the 7x50's are fine for low light use, as the large 50mm objective lens gathers a lot of light to brighten up the image before it gets to the eyepiece. The downside is that they are heavy, and everything else being equal, you pay more the larger piece of glass in front.
The smaller models like the 6x18's are more pocket friendly (smaller) and lighter, but they sacrifice light gathering power, so the image you see won't be as bright by comparison. They also have a smaller field of view, which is the area you see from left to right w/o having to scan the binoculars from left to right.
I suggest the size in the middle, either the 7x35 or the 8x32. Not too big, not too small, and depending on the brand you select, not too pricey, but still sharp enough so you can use it for hours w/o coming home with a headache.