Quote Originally Posted by mashamania
Do these nicknames or diminutives of Maria all pretty much have the same connotation? or is one or another more intimate or another more casual?
No, they're all different. I'll list all these forms from the most intimate to the most rude.

Mashenka is very very nice. You can call so a close friend, your child, your wife, or a person you dislike to make what you say sound cinical.
Mashula, Mashechka mean the same thing. Absolutely.

Masha is quite neutral, but you cannot address "Masha" an adult woman that is not a friend of yours. You use unformal "you" = ti with Masha.

Mariya is formal.

Mashka (Also Man'ka) sound familiar and slightly abusive. But you can call so a close friend of yours if you know she won't get offended.