I am new to after effects , and I just finished editing a video commercial that is supposed to run on an LED 3mx4m screen.
The video is quite basic and does not contain any 3d/special effects.
Now it's time to render the file.
what is the best resolution for it to run smoothly and good looking? also. is there a way for the file not to be to large?
Refer to the requirements of the player software/ hardware used to feed the screen and read up on basic rendering stuff.
Your approach is totally backwards. First - determine the size, frame rate and format of your final product (movie). Second, make a comp that fits those specifications. Third, render to the specified format.
You can only know what the specifications are for the LED screen. You say 3m X 4m so that could be a standard NTSC or PAL monitor or it could be a custom LED display that wants you to supply a file that is 1500 x 2000 or 3000 X 4000 pixels wide at frame rates from 12 to 30 frames per second. You can't know unless you ask.
As Mylenium said, you have to read up on rendering and deliverables. The default output from the Render Cue is a lossless file to be used again for further processing. Lossless files generally do not playback in real time and they are big because they are lossless. The most common delivery format is h.264 in an MP4 wrapper. You use the Adobe Media Encoder to create that kind of file. AE is a professional app and it requires a professional understanding of basic video formats, frame rates, codecs and standards. Video is not at all like still images that can be any size. Video is very picky...