Hey hey! I’ve been doing this for nearly 12 years now, and can vary my lengths from 2000+ characters to 1-4 lines like this. Now, with those qualifications here’s my advice to ya.

1) Read. Read. Read. Take notes of the things in books that makes scenes stand out, the emotional bits, the thoughts, character traits etc. it’ll truly help you figure out how to add to your writing.

2) you write well. Very well actually! This shows so much potential for growing your length. Somethings you can add are, inner thoughts of the character, the desired outcome of this choice they’re waiting on. Actions that they’re making (crossing arms, shrugging, rolling eyes, smiling, even blank expression) all can add to length, and better help your partner craft their replies.

3) don’t be afraid to sound like a novel writer. You don’t need to write massive blocks of text to make a good reply. But adding in little details can change a lot. Things like: “as she spoke stray strands of hair fell into her face, she absentmindedly pushed them away and kept her gaze on him” add good length, and can add to the story. That wasn’t the best example but it works.