Of the three acts, Act I, II, III, generally ACT II is the longest, ACT I the next longest, and ACT III the shortest.
Roughly, you can divide Act II in half, Act IIa and IIb.
And roughly, each will be 25% of the book. You could say the same about Act I and III, so all are equal. That is approximately my approach, within 5% or so.
I arrange my acts as follows:
ACT I: 25%. Introduce the Normal World. at about 1/2 mark, introduce an Inciting Incident. In the second half of ACT I, the inciting incident grows into a crisis (the big problem, in my mind) that forces the MC out of her normal world, by the end of ACT I, to deal with it. Either physically (she must travel to solve it), or metaphorically (e.g. she has fallen in love with somebody).
I am a discovery writer; so I don't actually have a plot in mind to begin, but before I type a word I do at least know my character, what her normal world is like, and what the "big problem" looks like. An example of an inciting incident, say for a love story, is something that causes her to meet the person she will fall in love with. For some other crisis, it might be witnessing a crime, or finding evidence of fraud going on at her company, etc.
ACT IIa: I like a method I've read somewhere of dividing ACT II, so the first half, IIa, is more of a "reactive" phase for dealing with the big problem. The MC tries some easy fixes, and they fail, but it teaches her something about the problem. Then:
ACT IIb: begins at the halfway point in the story, more or less; and our hero has an epiphany about what needs to be done, and begins a "proactive" phase of dealing with the big problem. Often this is a moment of change, or an end to denial of the crisis, or an acceptance of the loss she must take or unpleasant task she must perform. In short, she understands the problem, and now her actions, though they may still fail, are intentional and teaching her what she needs to know in order to succeed.
By the end of ACT IIb, she does know.
ACT III: Implement the plan, defeat the problem (whatever that means, it may mean breaking up with her fiance in order to marry the man she truly loves), and then live happily ever after.
I use this structure to guide me in discovering the story and plot as I go. I usually have the "solution" in mind as I am writing; but often the solution changes a few times as I am writing the story. If my original solution gets knocked into silly land by a story development I discover; I must find an equally good or better solution, or abandon my change and rethink.
The page limits are something I aim to stick with; I don't mind running over a bit, and if I do I will often do so at the expense of ACT III, forcing a faster paced ending. The page limits keep me from wandering or blathering too much in some ACT, they help me notice when I am doing that, so I can tighten it up on the fly.
For example, in a 100,000 word novel, I have 25000 words for ACT I, about 100 pages at 250 words per page (that is submission format). So I have 50 pages to introduce the normal world before I get to the inciting incident. If I am on page 30, and not even close to the inciting incident, I need to already look back and find some fat to cut or something to condense. Or if I get to the inciting incident on page 60, I need to cut a lot, probably some whole scenes, and lose the 10 pages. I write too much, so for me the sooner I know my book is getting fat, the easier it is for me to trim it. In each of these four segments, I am about on draft 3, with the right amount of pages and the right amount of "beats" (plot points) before I move on to the next segment.
In later drafts of the whole book I inevitably get longer, as I add color sense to descriptions, other sensory information, and improve the clarity of my descriptions.
That's my approach.