Well how I usually roll with keeping to strict AoE rules is with grids, there's rarely a reason to not just go "okay you hit them all" when you're just stating distances without a grid, so using that grid, I always made a bit of a pyramid with the tip being a 1x1 square next to the user. Then moving on to a 1x3 row, then a 1x5 row, basically adding 2 to each side as it goes on, until it's hit the appropriate length. For example, a Cone 5 Attack would have the base width be 9 meters.
I know it makes very little sense, but to be fair cone was one of those "weird" ones that I didn't really think through 100% when I made it back before I started working on DDA. I never really thought too much about it.
If you're not using a grid, or you want to figure it out manually for whatever reason, just assume that the base is equal to the height then for simplicity's sake, no need to go into crazy math to figure out the base for a equalateral triangle.