How to learn spacing?

[Kaiboiii]

I want to improve my spacing, how can I go about doing this? Practice with Marth maybe?

 

[Raijinken]

Practicing with Marth helps a lot, as does Charizard (his tail has sweetspots similar to Marth's sword).

Rosalina/Luma, Zelda, and Megaman are also rather spacing dependent, though not to the same extent as Marth is.

 

[Pale Tuna]

Best to practice with, like Raijinken has said, would be Marth. You can keep people at a fixed distance using his long-ish sword. Palutena is also a great character to try spacing with since she has a great reaching jab poke and her F-Air. Mega Man's lemons and aerials are also good to use for spacing.