It does matter for the query optimiser when it will evaluate the best query path to perform your query. When more than one path will be available, it will calculate an I/O cost and other various parameters based on your query and from these, chose the one that will appears to him as the least costly.
This is not an absolute calculation, it's only an approximation process. Therefore, it can easily be thrown off if the apparent mean size required to manipulate the records from one table in memory is much bigger then what will be really necessary and the optimiser might chose a less performing path based on what it thinks would have be necessary for the others paths.
Having a realistic max size is also usefull to any other programmer that will come along looking at your code. If I have a variable that I want to display in a GUI, I might allocate much more space than neededd if I see that is backed by something like nvarchar(200) or nvarchar(2000) instead of nvarchar(20) if its size is never greater than that.
