Dritermius,
You can use VirtualDub to split video files before converting to amv.
Load you video into VirtualDub from the menu bar select Video>Direct Stream Copy.
Audio>Direct Stream Copy.
Then File>Save segmented AVI.
When the save window opens you can se the size in MB you want to split the file into.
Then save, the whole process will only take a few seconds. Then convert the segments to AMV as normal.