Currently, there's no easy way to tell how large something is unless it is stated in the description or exists in real life so you can estimate it. If I make a custom creature, just like a Scroolex or whatever, that has no relation to anything found on earth, the problem is even worse, since just by looking at it there's no way of easily telling its size, since it's just a letter on a grid. And if you check the description you might learn a bit more, but even then it runs into the problem of size descriptors being a very relative thing. If I describe a Scroolex as large, does that mean it's the size of a dog or a horse? Is an enormous monster bigger than a gigantic one? The only way to really ensure the player knows exactly what they are looking at is to just clinically say "it is 70000 units in size" or something like that, which is what i do currently since it is important the player understands the scale of a monster they are facing in my opinion, due to how much it impacts gameplay. This however runs into the problem of just not being very pretty to read, which is unfortunate. And in the event the creature grows in size as it ages as an adult, such as dragons, description-based size explanations are suboptimal since you cannot readily tell how old a creature is unless it is a member of your fort or you happen to have a duplicate save in which you can access legends mode, either option being a chore that drags out the process of a single fight by the player having to leave it paused until they can figure out whether what they are fighting is a horse-sized whelp or castle-sized monstrosity.
I think something added to the secondary bit of the creature description which handles procedurally generated parts like proportions and coloration, which just says something along the lines of size relative to you (it is about your size, it is twice your size, it is ten times your size, etc, maybe getting vaguer as the differences become more apparent like "it seems a few hundred times your size (100-200)" "it is hundreds of times your size (300-600)" "it is many hundreds of times your size (700-1000)" or something like that) would be a useful quality of life improvement.