It’s been eating at me for ages and I just never bothered to puzzle it out, but when typing Japanese, to get the smaller characters like small tsu that indicate a doubling of a consonant (like tt, pp, kk, etc.) just type your Japanese word in Romaji with the repeated consonant sound twice followed by the vowel and (hopefully) one’s IME should show the small tsu with the correct follow up character.

So for “pretty” as kekkou, type ke-k-ko-u which transforms to けっこうautomatically.