Nearly all letters I get are in C5 envelopes, i.e. A4 folded in half. But Denmark is so electronic that "nearly all" is about one a month.

Folding in thirds isn't worth the bother when I post things so infrequently.

Choosing a large British supermarket, they have C4, C5 and C6 sizes, plus DL (A4 in thirds) which are cheapest: https://www.tesco.com/groceries/en-GB/search?query=envelopes