I have downloaded a map from the internet and added it into my saves but when I try to to open it minecraft stays on 0% loaded for a few minutes then crashes. Could this be because of the map size (9.6gb) or the fact the map was made in 1.12?
You should only play maps in the version they were made for; it may be possible to "optimize" the world (in 1.13 or later) so it can be played without any of the performance issues caused by having to convert chunks on the fly.
Otherwise, aside from MC-33134 world size doesn't matter - the game only loads chunks within the spawn area and your render distance so you only need as much memory as a new world with similar terrain would require (note that you can simply delete the "worldname\data\Mineshaft.dat" file with no impact on the game since they have no unique mobs (excluding mob spawners) or ways to locate them).
Either way, a crash report (.minecraft\crash-reports or the game output in the launcher; enable under settings if necessary) would help pinpoint the issue, unless by "crash" the game simply stops responding (if you leave it for a while it may eventually crash, this may happen if it runs out of memory).
I have downloaded a map from the internet and added it into my saves but when I try to to open it minecraft stays on 0% loaded for a few minutes then crashes. Could this be because of the map size (9.6gb) or the fact the map was made in 1.12?
You should only play maps in the version they were made for; it may be possible to "optimize" the world (in 1.13 or later) so it can be played without any of the performance issues caused by having to convert chunks on the fly.
Otherwise, aside from MC-33134 world size doesn't matter - the game only loads chunks within the spawn area and your render distance so you only need as much memory as a new world with similar terrain would require (note that you can simply delete the "worldname\data\Mineshaft.dat" file with no impact on the game since they have no unique mobs (excluding mob spawners) or ways to locate them).
Either way, a crash report (.minecraft\crash-reports or the game output in the launcher; enable under settings if necessary) would help pinpoint the issue, unless by "crash" the game simply stops responding (if you leave it for a while it may eventually crash, this may happen if it runs out of memory).
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?