setup size of lines

[Carlos Timóteo]

i have 5 line shapes whith roads, rails, boundaries etc. and i want to
setup different lines sizes to make it more visible,

because with all sizes equals, only with different collors, is very
hard to understand the information.

i have different classes for each feature with 

different sizes, 
minfeatsize=1,
maxfeaturesize=100 

but all lines are draw equal.

there is any chance to setup line thickness ??

Thank you in advance.

Carlos Timoteo, Portugal