The absolute minimum should be 500x400 as there are ads this size you wouldn't want to miss. As mentioned earlier, 640x480 is ideal.
Size, the lighter the better, never go beyond 10 MB, aim for 5 MB. Use global objects and reuse them, also learn how to compress your audio files so they sound good but are not too heavy, those are the ones that take most of the space.
When you are debugging your game try to have less than 100 active objects per frame, it may run well as an EXE but when converted to Flash the active object overload will slow it down. If you have a large frame level try to split it into several sections to fix this.

