Since none of this is answering the question....
A narrower channel drains the pipe slower but will start evaporating sooner...
Sigh. Lies. It appears Cog hasn't read the wiki either. If you're going to answer, answer correctly, so you don't spread confusion, leaving others to clean up your mess.
OldSkool - Magma will
never evaporate from a canal of 1-tile width. Why? Because it's filling, and any magma that is 1/7 will almost immediately become 2/7, and the next tile will then become 1/7. To evaporate, the 1/7 depth has to sit around for a while - and that simply won't happen.
What's more, if it's a pipe (not a pool), it will refill. If it's a pool, a 1-wide tile tunnel drains less. 1-wide is the way to go if you don't need a massive flow for other purposes (making obsidian, death traps, etc.)
You can dig a feeder-tunnel all across your map - it might take a few "days" to fill, but that's usually fast than digging something wider for no purpose whatsoever.
And if I vented momentary frustration, I also answered the question. You
don't have to worry about magma cooling, nor evaporating in this instance. Since it doesn't cool, you can pipe it forever.
We could talk for 10 paragraphs about magma and not include every significant consideration. The wiki has all the above info, and more.