[name_f]Autumn[/name_f] is 99.9% likely to be the first name.
I think [name_f]Niamh[/name_f] flows better but [name_f]Nia[/name_f] is shorter letter wise. I’m planning on using more than one middle name, so [name_f]Nia[/name_f] having fewer letters would take up less space on forms. Does [name_f]Nia[/name_f] sound too weird/off with the first name? There’s no plan to use the two as a double barrel name so she likely wouldn’t get her middle name used aloud unless she was in trouble. 
I do slightly prefer the flow of [name_f]Niamh[/name_f], but I don’t dislike [name_f]Nia[/name_f] with the first. I think the only thing that throws it off, is that I feel like I’m about to say autumnal then messed up – but, given that it’s not going to be used as a double name and if you were saying both, it would be clearly enunciated if she was in trouble 
So, if using a shorter name feels more practical, [name_f]Nia[/name_f] works (and it is charming) – though I would say, unless your surname and the other middle/s your choosing are really, really long, I don’t think [name_f]Niamh[/name_f] will take up an excessive amount of space on forms?
I like [name_f]Autumn[/name_f] [name_f]Nia[/name_f] better than [name_f]Autumn[/name_f] [name_f]Niamh[/name_f] personally
I think they both sound fine with [name_f]Autumn[/name_f]! Personally I like [name_f]Niamh[/name_f] more as a name, but it would probably depend on the second middle as well in terms of which I preferred in the combo. Unless the second middle is really long I don’t think the extra 2 letters would be too much of a problem!
I was thinking of using my middle name which is seven letters.
I love [name_f]Niamh[/name_f] as a girl with [name_f]Irish[/name_f] blood running through her veins [name_f]Niamh[/name_f] appeals to me greatly. She’s elegant, sparkly and bright. I also love the meaning of [name_f]Niamh[/name_f] especially paired with [name_f]Autumn[/name_f]. ‘Autumn bright (bright autumn)’ I think she’s gorgeous. I strongly prefer to [name_f]Nia[/name_f]. I think [name_f]Autumn[/name_f] [name_f]Niamh[/name_f] is beautiful. Regarding form filling as someone whose daughter has a long name (first two middle names two surnames) this has never been an issue for [name_f]Lilia[/name_f] and I wouldn’t imagine would be an issue for [name_f]Autumn[/name_f] [name_f]Niamh[/name_f] (second middle surnames) go with the name you prefer.
I’m leaning towards [name_f]Autumn[/name_f] [name_f]Niamh[/name_f] for sound and because you prefer it. I think the deciding factor for me would be the flow with the second middle. I don’t think 2 extra letters on a form are going to make that much of a difference.
[name_f]Autumn[/name_f] [name_f]Niamh[/name_f] sounds the best for me, but I agree it depends on the second middle as well 
[name_f]Autumn[/name_f] [name_f]Niamh[/name_f] flows so much better! I also don’t think having a 5 letter middle and a 7 letter middle too excessive. [name_f]Autumn[/name_f] [name_f]Niamh[/name_f] feels so crisp and sweet, I love it for you!
I think the problem on forms is not the length of the middle names, as much as there’s often only a box for one middle name or one middle initial. So if you’re going to use multiple middle names, just use the ones you like.
[name_f]Autumn[/name_f] [name_f]Niamh[/name_f] is gorgeous! I think it might depend on the other middle(s) you plan on using - something like [name_f]Autumn[/name_f] [name_f]Nia[/name_f] [name_f]Claire[/name_f] flows better than [name_f]Autumn[/name_f] [name_f]Niamh[/name_f] [name_f]Claire[/name_f]? But just gut reaction, [name_f]Autumn[/name_f] [name_f]Niamh[/name_f] has my heart. Good luck!