If you look at the name of the game Dungeons & Dragons and compare it with the reality of pen & paper roleplaying, it is obvious that the Dungeons part is far more frequent than the Dragons part. Over an average full campaign you will have many dungeons, but only few dragons. One reason for that is that "dungeons" is a far more open term than "dragons", there can be millions of different dungeons. If you need a dragon, you just take one from the Monster Manual; if you need a dungeon it usually is worth putting some effort into creating one. So how does one make a good dungeon for 4th edition D&D?

The first thing to realize is that 4th edition D&D has some substantial differences from other editions, which is cause for a lot of edition warring. But once you leave the squabbling behind you, the differences also have consequences for dungeon design. Basically earlier D&D editions (and D&D Next) had more and smaller fights, while 4th edition tends to have fewer and bigger fights. Thus an earlier edition dungeon might first have a small room with 3 kobolds, then another small room with 3 goblins, and finally a room with the one orc chieftain leading them. The 4th edition version will rather have one big room with 3 kobolds, 3 goblins, and the orc chieftain all present at the same time.

Thus old school "blue" dungeon maps of first edition AD&D tend to have a large number, sometimes of over 100 rooms. And each individual room would be small, with the dungeon map usually just showing an empty room. Furniture or other features might be mentioned in the text, but wouldn't appear on the map. And in some extreme cases you'd open a door to a 3x3 room and find 10 orcs in there and nothing else, because nobody bothered to wonder what those 10 orcs were doing in an empty room.

4th edition dungeons need a bit more effort to create, because a room with monsters in it is at the same time a battle map for that encounter. And terrain plays a huge role in combat, so your dungeons get a lot better if the rooms aren't just empty. You will want to know not only what features are in the room, but also how the monsters are placed in relation to those features. An archer behind a barricade at the back of the room is very different from an archer standing on the first square in the open after the door. Therefore it makes sense to build 4E dungeons from the bottom up: First create each encounter with its battle map, then connect the battle maps to form a dungeon. In earlier editions dungeons were often created top-down, starting by drawing the dungeon on a squared paper and then filling the rooms.

While I still use squared paper for the first sketches of a dungeon, I've moved to Campaign Cartographer for the final map. In Campaign Cartographer you can save zoomed in "views" of one room and just print that view out as a battle map. That avoids showing the players the whole map: The view of the room they see is at the same time a description of what they are seeing, and the battle map if a combat takes place. But I don't just print the views of rooms in which combat is planned, but of every room. That has the advantage that putting a map on the table isn't a sure sign to players that there is a combat ahead, and also serves well if a combat for some reason moves out from its room.

For some of my larger battle maps I went to the trouble of having them printed as a "poster", most online poster shops have a service to print your images on a large piece of poster paper. But that is obviously expensive, and with the usual 1 inch equal to a 5 foot square, most battles don't really need a huge poster. Using A4 paper, I try to fit smaller room on one page of 8 x 11 squares, medium rooms to two pages of 16 x 11, and large rooms to 4 pages of 16 x 22 squares. Anything bigger tends to get messy if you try to print it at home and then tape the pages together.

Another nifty feature of Campaign Cartographer is that it has layers, and you can put some dungeon features like secret doors on a special "secret" layer. Then you print the zoomed in battle maps with the secret layer hidden, while the overview map for the DM shows them. In some cases I print out the secret features extra, and overlay them on the battle map once the players have found them.

The ideal size of the overall dungeon basically depends on your play rhythm. My group only plays twice per month, with sessions lasting 3 to 4 hours. Thus my dungeons are more likely to have something like 20 rooms or less, with around half of these being battles. That way my players don't spend half a year in the same dungeon. But if your groups likes long dungeon crawls or plays more frequently and longer sessions, nothing speaks against making much bigger dungeon maps. Just remember that dungeon size also has an influence on the narrative of your adventure: Many rooms with many fights against similar monsters might slow story development down to a crawl while the players are busy with one fight after another.