It really does depend on what you're going for. There are 5 landing beaches and a ton of paratrooper landing sites involved, as well as other POI's like Point Du Hoc, Pegasus Bridge, Brecourt Manor, St Come Du Mont, St Mere Eglise, Carentan. If you're trying to make it historically accurate, there are tons of detailed maps, resources, photos, and videos to model your map off of online. It is one of the most documented military operations in history so there's no shortage of information. Games like Forgotten Hope 2, Squad 44, and I think Hell Let Loose (never played it so I can't say for sure) have pretty close recreations, FH2 in particular models all 5 beaches 1:1 scale in exact detail, including point du hoc and pegasus.
Do no, I repeat, DO NOT, use Saving Private Ryan as a source, nor Company of Heroes, nor Medal of Honor Allied Assault or Call of Duty WW2. They are all based off of SPR, and that was filmed at the training beach used to train the Allied landing forces of Operation Overlord, Saving Private Ryan does not have an at all accurate layout of any section of Omaha.
This is, of course, if you want your map to be accurate generally. There is nothing stopping you from taking creative liberty or design a map more around gameplay if you wanted to. In Minecraft you generally have to scale things down too because there's a limited render distance and entity render range (unless you use the right mods *cough cough* shameless fob online plug *cough cough*) so do keep that in mind when deciding on the layout.
