I'm not a big Marth player, but I know a few people who have some nice spacing.
Spacing for me is keep the right distance from your opponent so you aren't in the danger zone of their moves. Marth has a nice range because of his sword and his extremely broken grab range for when you get close enough.
To space with Marth, I usually do a lot of fairs and nairs because it gives such a great distance that your opponent cannot be in. If they do get hit, you can easy wavedash into a grab or keep your space in order to keep punishing them. Another way to simply space is downtilt when your opponent is close/off of the the ledge or you're on a flat surface like battlefield. The downtilt keeps you low to the ground to avoid some attack and keep crouch cancel and it has a great range and is an excellent shield poker.
Like I said, I'm not really too big on Marth, but this is some stuff that I've picked up.
