Setup/Compatibility
The Compatibility window allows a couple toggles on very strict emulation of the limits of the original games:
Vanilla savegame limit determines whether the savegame size is limited or not. In the original game, the limit is around 180KB and may be exceeded with very large levels. If the limit is enabled, the game may crash when attempting to save a game.
Vanilla demo limit determines whether the demo size is limited. Like savegames, demos were limited in size in the original, to 128KB (around 15 minutes of solo play). Although the size may be increased per recording using the -maxdemo parameter in the command line, the limit may still be reached on extremely long play-throughs (such as a full max run in a megawad) and likewise may be desirable to disable to avoid a crash while recording. This toggle does not affect playback of demos of any size.
Most players will probably want to disable both of these limits.