I am facing player with decks up to 100 cards! I mean, how can you build a consistent deck with 100 cards? how much mana do you use in a deck like this? Is worth having a 100 card deck? I am interested to know your opinion about it.

a deck with 100 cards is necessary less consitent than a deck with 60 cards although possibly not by as much as you might think.
how much mana you want depends on the curve of the deck but probably around 40.
a 100 card deck will generally be close to strictly worse than a 60 card deck.

You know there are entire formats built on 100 card singleton decks? Consistency is not the most important part of deck building to some people.

Formats like 100 card singleton are a different animal, as everyone is playing fat stacks. Context here is clearly if they are good in Duels, as the first question is about consistency.

Of course you can build whatever size deck Duels permits for whatever reason you like. But if your concern is winning you should stick to 60. More cards = lower odds of drawing what you need. There’s a reason you don’t see fat stacks in the top 8 of pro tours.

But yeah, if you just want to goof around, go for it. NightHawk built a 100 card all-mythics fat stack for an April Fools video and won a game with it. It was pretty hilarious, and likely a fun experience for him. But if you’re trying to hang out in the high ranks, it’s not going to keep you there.

There are at least two potential benefits, to be fair.
Better tutoring and protection againts mill.

Is it worth it? Prooooooobably no.