A number on a drawing tells you nothing until it stands next to something you have held. So here are our two objects against four things already in your house.
Every bar is a real length. The two orange ones are ours and they come off our own drawings. The dashed lines are every 50 millimetres, so the whole thing works as a ruler and not just as a picture.
Three hundred and eighty millimetres nose to tail. That is about four and a half credit cards laid end to end. Its wings are 340 mm across, which is wider than this page is tall on most phones.
Pick something on your table. Do not measure it. Write down what you think it is in millimetres, then go and measure it, then write down the difference.
The difference is the interesting number, not the guess. Do it ten times and the difference gets smaller on its own, which is the whole of how anybody learns this.
One inch is 25.4 millimetres, exactly, by definition. Every bar below is labelled in both because a nine year old in Wisconsin thinks in inches and a drawing does not care what anybody thinks.
ISO/IEC 7810 ID-1, 85.6 mm
US currency, 155.96 mm
US Letter, 11 inches
30 centimetres, by definition
Max tech pack, dimension A (bulb-dependent, so it is a reference)
Jumbo tech pack, dimension A
Dashed lines every 50 mm
Both of ours are reference lengths off dimension A of their tech packs, and Max's depends on a bulb nobody has found yet, so his number can still move. Nothing else on the ladder can: a credit card is 85.6 mm because a standard says so.
That is the end of this one
The word for what you just did is scale.
Scale is knowing what a number feels like in your hand before you have built anything. It is the difference between a drawing you can check and a drawing you can only read. Everybody who is good at this can look at 380 mm and know roughly what is coming.
Where to go now