I really love the name [name_f]Marcy[/name_f], but some people think it doesn’t suit a grown woman. I think it’s alright, but what do you guys think? I really don’t like giving a child a name only to just call them a nickname, but is [name_f]Marcy[/name_f] just too childish?
I don’t think it’s too childish. It feels like other similar, complete names: [name_f]Macy[/name_f], [name_f]Mary[/name_f], etc.
My [name_f]MIL[/name_f] is a [name_f]Marcella[/name_f] but only goes by [name_f]Marcy[/name_f], so I can easily see [name_f]Marcy[/name_f] being on an adult. I don’t think you’d need to give it a full name.
I think [name_f]Marcy[/name_f] is fine as its own name, but [name_f]Marcella[/name_f] is beautiful as well! I think either option is great.
I prefer the simplicity of [name_f]Marcy[/name_f] and think it would age just fine!
I think [name_f]Marcy[/name_f] stands on it’s own perfectly fine. I know several Marcys, but I never assumed they had a long form with it. I think of it like [name_f]Lucy[/name_f]–it can be a nn but it’s just as fantastic on its own.
I do love [name_f]Marcella[/name_f], so I’d love for you to use it for my own vicarious purposes 
Another option for full name, if you decide you want one, is Marcelline/[name_f]Marceline[/name_f].