It adds some items, recipes to which you can see with JEI/NEI.
These items are pretty self-explanatory, but to get the recalibrator that makes you normal size again, you need to deplete any recalibrator(so the cheeper one you make - the more you click).
* Liquids are funky
* Ladders are funky
* Any player projectiles are funky (at least when you small)
* No riding(but will be in the future)
They can corrupt worlds just like any other mod can (except that maybe they have just a bit more tools for that). My mod does no specific interactions with your saves (just regular NBT to store your size as a number - all mods do that kind of stuff).
They DO NOT EVER corrupt you jar-files, the whole point of them is not to modify the jar but still modify the program at the point of loading classes from jar-files into RAM.
They MAY not work just for you for unknown reasons, but most likely they will.
If you have any issues, feel free to report them at issues page, but they only will be reviewed if you provide your fml-client-latest.log file.
You can use this mod in your modpacks freely. To be nice you could also tell me something like "added to modpack XXX" but this is not required.