You cannot define the size of an input field. But have you tried placing it inside a table, that way you could establish the width of each column. For example:
If you were already using a table, then the problem could be that other columns are empty. And in that case empty column won’t be created.
To prevent that you can see in my example I added this non-breaking space chracter ( ), so the column is no longer empty and it’s created (but you won’t see anything in it since it’s only a white space added).
I have raised this issue with our design team as this is not something that can be fixed in liquid at the moment. We will update you when we have an answer.