OK, a quick techie overview about field sizes...
All fields are stored pretty much in the order they are defined. So in theory, the first field you name is stored first, the second field you name is stored next to it, and so on. Numeric fields (yes/no, integer, real, date, currency) have a fixed size and are stored directly in the record using the number of bytes needed to hold the maximum possible number for the given field type. So if you have a LONG integer, you need 4 bytes to store +/- 2 billion. If you have a DATE, you use 8 bytes to store dates from 1/1/0100 to 12/31/9999 and times from 00:00:00 to 23:59:59 in one combined slot. Yes/no fields store the minimum size of 1 byte. Each numeric field's contribution to the internal record length is well known.
For short-text and long-text fields, the story is a little different. For text fields, the field size defines the MAXIMUM number of bytes that can be stored in the field. If you try to store more than that, you get a constraint violation. It is OK to store fewer than the maximum number of characters, and the field can even be empty (a zero-length string).
In the stored record, a string field has a fixed-size pointer structure at the position of the field. For short-text fields the pointer identifies a spot following the fixed-size part of the record and also shows how many bytes were ACTUALLY stored. So the contribution to the size of the record is the size of the defining structure for each string PLUS the number of bytes actually stored for that string in that record. For long-text fields, the pointer structure is bigger than for short-text strings because long-text strings are stored separately from the record to which they belong and is not necessarily stored in the same area as the owning record.
The story doesn't end there, though. For forms and reports, particularly those built by one of the wizards, the string field size defines how much space to allocate on-screen or on-print for that field. For import operations, that field size identifies truncation points if importing from some source that might have more information than you wanted to keep. For export operations, the field size identifies how much space to build into the string that will represent the external record.
File size will not change based on whether you change the maximum size of a field. It will change only based on how you occupy the field.