For AVI:
The output dimension is always the same as the file you are converting.

For MPEG:
The output dimension depends on the type of MPEG you want. For instance, if you are converting your file to write to a DVD then you may select MPEG-2 DVD (NTSC) or MPEG-2 DVD (PAL) depending on the picture standard used by you. This shall create a standard complaint output file for DVD burning. To select your own dimension (non-standard), select Custom MPEG-1' or Custom MPEG-2' and then specify the required dimension.

Remember that custom MPEG-1 or MPEG-2 dimension are not standard and you cannot burn them on a VCD, SVCD or DVD. However, you can play them on your computer.

For Windows Media 9:
The output dimension depends on the selected profile. If you want to publish your converted file to the internet then you must select a profile that enables your target audience to view them at their internet connection speed. The profile defines the dimension and you cannot change them.