If you go to the "Output" section of the Preferences, you can set a value at which your movie files can be segmented.
Just out of curiosity, what codec and frame size are you using?
Prefs setting will split the video. There's no auto way to set your movie to a specific file size. I guess keep exporting with different quality and bit rates.
This task seems better suited to a separate encoder like Adobe Media Encoder, Squeeze, or Apple Compressor.
Is the output file size an option in render settings?
No, there is no such setting. Even more so, as AE is not the ideal tool for controlling the size of compressed output. The program itself has no concept of multiframe/ multipass compression and the specific output modules for H.264,FLV or MPEG-2 are limited by that as well. In addition, whenever asking for specific file sizes for web output, you enter the treacherous waters of more generic problems with quality vs. resolution vs. framerates vs. audi compression vs. file size, which is a topic you could write an entire Encyclopedia Britannica about. For your purposes, I recommend you render the segments to an intermediate visually lossless (Quicktime animation) or only mildly compressed format (Quicktime PhotoJPEG), then let the standalone Adobe Media Encoder or any otehr tool of your choice handle the final encode. Another option would be to render to H.264 files directly from AE, then use SUPER©, ffmpeg or MPEGStreamclip to do "quick transcodes" to find a good balance between data rates and file sizes for upload.