Through-hole tech is very nice for hobbyists. 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

Those 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 inches. There is also a metric equivalent, but the imperial numberings seems prevalent.

Semiconductors

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 cheat sheet that can be printed in 1:1 scale.

nice SMD sizing cheat sheet