Cost of using over dimensioned columns

[Morten]

Hi list,

I'm wondering what the "cost" (if any) is by using eg. mediumtext  
over  text, or largetext over mediumtext. Is there a non-negligible  
reason to not just just  largetext?

And how about varchars, what's the "cost" of using varchar(1000)   
rather than varchar(20)?

My intuition tells me that I should keep things as tight as possible,   
but I may be narrow minded and need a sanity check.

Thanks!

Morten