Sooooooo, can anybody confirm whether VLC supports files bigger than 64 GBs or not? I can't seem to be able to play one that is 64.4 GBs big.

Oh, and, any alternatives to VLC? (looking for players that don't change system codecs)

Just curious, but what is generating a 64.4 GB MKV file?

I'm also curious as to what exactly generates a 64GB file, even Bluray is something like 50GB max I think
As for another player, I've been using KMPlayer for a while now and it's quite nice. Not perfect, but perhaps a reasonable alternative.

mplayer2
ffplay

you could just split the file with mkvmerge
mkvmerge input.mkv --split size:32g -o output.mkv

I'm not familiar with mkvmerge, so forgive a potentially dumb question, but wouldn't that split his file into 3 files, 2 32GB files and 1 0.4GB file? Might as well change it to --split size:33g to make it more manageable (assuming the VLC limit is 64GB, but I actually don't know that either).

I don't know what I was thinking when I wrote that. You're right, that would create 3 files so splitting at 33GB would be better.
Even better option would be to use timecode to split at scene change (mkvmerge input.mkv --split timecodes:HH:MM:SS.nnnnnnnnn -o output.mkv). Unless the file is used as a source for video editing in which case it doesn't matter where you split it (the big filesize would indicate lossless source file for video editing purposes)

Lord of the Rings: Return of the King (the extended version). Ripped and merged from two BDs into one MKV file.

EDIT: I wish WMP supported .MKV out of the box, without installing additional stuff. :(

Thanks for all suggestions so far! I'll try them out tomorrow.