Mouse acceleration reduces the sensitivity at low speeds and increases it at high speeds.
It is designed to assist with low resolution mice so that you can carefully move a few pixels when editing an image or move from one side of the desktop in one sweep.
Gaming mice have much higher resolution, and you will find the cursor tracks far more accurately with mouse acceleration disabled.
While on the topic of resolution, the default for my version of the Deathadder is 1800 dpi, with a maximum of 3500 dpi, and there are versions with 6400 dpi. I find anything over 1800 dpi is too sensitive.
I think my mouse mat is the standard version. This is just the look of the mat anyway, so pick the one you like.
I chose the control version over the speed version. It feels rougher, but gives you better control (hence the name).
The medium size gives you plenty of room to move the mouse. The larger mats will end up at least partially under your keyboard. The small is like a standard cheapo mouse mat size.