The game's sizes are not in meters but in blocks with 1 block approximately equal to 0.5m which is the same as about 20in.
When using the size command you enter the block size not the size in meters e.g. size 1 1 1 on a wood beam creates a block sized cube i.e. a cube with a = 0.5m = 20in
Indeed. But manually doing that conversion every time is tedious. This simple plugin would solve that by generating the correct values and running the command. Even better would be if the plugin had size presets, so that all you have to do is click a button.