Often times, I will have their response on one screen and Word on the other
This is a very solid tip, especially for those of us who write lengthier posts back and forth. It also makes editing your response much easier should you feel the need to do so, which I often do.
On a more general note I agree that you have to realise that when in a roleplay you are not able to control every aspect of the storyline and that in itself is at least half the fun but it also means that you have to leave openings for your collaborator/s to evolve their characters, to move the story forward. Even during conversation people do things, small things, perhaps even relatively unimportant things but as they saying goes, the devil is in the details. During conversation a character can either move from one place to another within a confined space, say for instance get up and start making coffee, look out the window and notice something, or if the characters are moving from one place to another while having a conversation I often like to describe things along the way inbetween lines.
Length as such is not as important to me since it can often vary depending on the context.
It is also important to stay true to the previously agreed upon settings for the roleplay and not add things without first discussing it with your collaborator/s I have sadly had far too many experiences with players who agree to a certain set up only to then, once the roleplay is underway, change everything to their own preferences, or worse, start to play MC, moving them about, what they think, what they say in dialogues and so on.
Another important thing is to not write too far ahead leaving your collaborator/s with too many actions to respond to since as we all hopefully know, each action has not only a re-action but a consequence which may or may not change the context of the next action and re-action to be taken. To give your collaborator/s too many actions to react to can be restricting to their creativity as they have to match their re-actions to match the next action you have written.