How do I call a function from a user command? (Getting a "trailing character" error, E488)

So I was trying to setup a custom command to change tab width on the fly.

function! SetTab(width)    set tabstop=a:widthendfunction:command Tab -nargs=1 call SetTab(<args>)

The idea is to when I type :Tab 2, the current tab width will be changed to 2.

But it doesn't work and gives E488 Trailing characters when I issue the command, not sure where it's coming from, I couldn't find any trailing char.