As nobody reacted to that for almost 4 years now and because that is exactly what I would want to see implemented, then here it goes: I am resurrecting this lonely request of @Oblio. And if I can make my case:
We have now bigger monitors and higher resolutions available- so at least some of us can now utilize more space as they simply have that space available within various software. I know that is my case- after switching to new much bigger screen I gained more space, even in spite of telling my OS to display bigger font. [And one of the reasons for that switch of mine was the way to small amount of data that I would see in Mp3tag on that old monitor]
This could also be a workaround-answer to one of those two features 2 Ideas: Comment hover, Cover display window size suggested by @Mikeez
I just really do no see why should not the Multilne be definable by the user. But of course the best solution would be to make it possible to define the number of lines for each of the fields - and not just for one overall Multiline. Or if it would be too difficult / buggy, make at least some choices available, like 5,10, 15 and 20 instead of that current just-1-or-6 option