This is a continuation mod for https://www.curseforge.com/minecraft/mc-mods/block-meter which seems not to get updated by its owner any more, forked under MIT license.
It allows you to measure sizes of builds or areas so you can verify things line up the way you want, or having the distances they're expected to.
To use, hold any item and tap the BlockMeter key. Right click that item anywhere to set a starting point. Right click the item somewhere else to get a second point. BlockMeter will draw a cuboid and show the lengths of each of the sides.
Tap the key again to remove all BlockMeter cubes. Tap Shift and the key to remove just the last cube.
From version 1.4 on, you can collaborate and see each other's boxes if the mod is installed on the server as well.
The mod will still work client-side only, so you can play on any (vanilla, spigot, paper, ...) servers without the server owner having to do anything.
However, in a LAN world with multiple players having the mod, or with a dedicated Fabric server that has the mod, you can see other people's boxes - enable BlockMeter, press the ALT button while holding the BlockMeter block, and toggle the new button. Now you'll see other players' boxes, and each block count will have the name of the player who created the box in front.
This only works when the server and all players have at least version 1.4 of the mod.
Version 1.4.1 works as server part with Bukkit/Spigot/Paper as well. If you want to share block outlines on your favourite server (non-modded, but has plugins), ask your server owner to download the jar file from here and put it in the plugins directory (make sure it's the correct Minecraft version, i.e. 1.15, and at least version 1.4.1 of the mod). The client part is the same as 1.4, so there's no reason to update your client.