Check out these tools:
AviDemux - Video software and downloads - VideoHelp.com
MKVtoolnix - Video software and downloads - VideoHelp.com
splitMKV - Video software and downloads - VideoHelp.com
Hi,
splitMKV is a command line tool. You need to run it from Command Prompt. Here's a tutorial on splitMKV:
splitMKV - split big MKV/AVI files + external SRT subtitles in smaller chunks (FAT32) - Doom9's Forum
And, here's one more tool, in case you need it:
mkvtoolnix -- Matroska tools for Linux/Unix and Windows