Is it really a problem? I kind of don't mind this.
Well, it's up to you! I hated it in my project; it looked turbo unprofessional to me. Like, you'd never see that happen in Zelda or whatever, you know? I like to make things look as slick as they can. But if you don't care you don't care
Keeping the number of character per line is a good idea, but do you think it would still be possible if I allow to change the text font?
You're right in that you do need to have your characters per line locked down to a constant number for this to work, which is a reason (one of many) I use spritefont objects for text, and not the actual text object, because yes, changes to font (and also differences in how fonts display on different systems) will mess up your count of how many characters to allow per line. I would really recommend using spritefonts for dialogue boxes instead of normal text anyway, aside from the issue of the typing effect - if you're making a serious game that will need to work on a variety of different machines, the normal text object is just not a robust enough approach to handle it.