Sizes of SMD Components Explained
Through-hole tech is very nice for hobbists. But it is getting obsolete. And many components are not available anymore in anything else than SMD. The main problem is that there isn’t a single standard size anymore
Passive Devices - Resistors and Capacitors
Thoses are very simple. They are rectangular, and the package naming is a number that directly reflects its size. For example, 1812, 1206 and 0805 refer to measurements in hundreds of centimetres.
Active components on the other hand, are a mess. But Vishay has a nice document on SMD package sizes.
STUDIOPIETERS® has even a nice SMD sizing cheatsheet that can be printed in 1:1 scale.