I noticed whenever I make maps sometimes they turn out too small and sometimes too big. So do how do you know when a map is too big or small AS YOU'RE BUILDING IT? I want to know because I am trying to make arena maps for Team Slayer, CTF, MLG, BTB, etc. So are there any tips to making sure it's not too big or small?
I go into player mode a LOt, and run around and get a good look at what I've just built, every few pieces, every piece... depending on the situation. make it so the scale "feels" right when you run around, unless you're trying to do a perfectly accurate remake or something. I am making a level based on a popular television tv show's set, and doing the scale so that it is comfortable to move around in the rooms certainily is much larger than the original. Remember that the player in halo runs faster and jumps higher than many othergames, so things can be a bit larger for that reason.
Go into player mode a LOT and run around...get a feel for what it'll be like if you were to actually play. I made a map inspired by maps like Lockout, Guardian, Ascension, etc. and kept going into player mode to run around and see if it'd feel similar to those maps without being identical. I'll be making a BTB map soon and I'll do the same. That's all I can really say.
I ran into the same problem when I started forging in Halo 4. It's simple really, experience. You eventually learn depth and scale of objects over time to where having to spawn in the map becomes less needed over time. Just remember roofs should be slightly higher than jump height @ minimum and if you're incorporating vehicles, make sure the paths are wide enough and roofs are high enough by testing them. Mantis stands very tall so it needs high clearance. Tip, any major area/room should be big enough to house a four person firefight. Not a rule but something to consider. Last tip, just spawn a hog to go off of for size starting off so you understand scale better and don't have to constantly jump into player mode.
Before I start forging I get a general size measurement of other maps. Just place down a hill marker and measure a pre-existing map of the size you would like yours to be and that will give you a general idea in size. An easy reference is Narrows for me and it's about 56 long and 20 wide, height is up to you (definitely a rectangular designed map). Once I have a general size in mind, I go in each forge world, create that size hill and drag it around all three forge worlds with that palette in mind and consider where I want my map placed. I feel the location of a map very important as it creates a specific environment around the map. If I can stress the most important things are to use objects the way they are designed and not to over build an area. Build floors, then walls, then lines of site, spawning and at the very end add decorative items to not exceed your budget. Also when building a symmetrical map, be absolutely sure to build a from the middle, building a little of both sides each as you go using coordinates mostly. Your center point should be all numbers rounded off to a .0 for the starting points (Height, Depth, Width). This makes it tons easier to build and it comes out a lot cleaner as well (magnets do not place items evenly or exact...) Hope this helps.