I can’t find the keyboard shortcut to みっ - Can anybody let me know how to type this in to the UI? I don’t see it on the chart anywhere either.
To make the small tsu, you can always double up on the next consonant.
みっつ mittsu
がっこう gakkou
Additionally, if you really want to make a small tsu on its own, xtsu or ltsu can do that. A leading x will make a small version of various characters.
Oh, and looking at the chart, small tsu is listed under 雑多 (miscellaneous)