Is going 25' going to be too much for a rookie?
If ya' say "no," it makes it sound easier than it is. If ya' say "yes," you negate the fact that plenty of people have done it before.
I think a boat (power or sail) over 24' is considerably different than a boat (power or sail) under 24'. There's probably another break up the length scale even further, but I don't know where. All of the elements - size, mass, hull style, etc. - just work different. So, there's that consideration.
It's a sail boat, and not a power boat. Again, just very different type of operation. Plus you're going to trailer it, so you'll likely have a swing keel or shoal keel - and that's even different. Not to mention trailer launching, stepping the mast and rigging the sails every time you wanna' go out. So, now you have two aspects to learn going straight to a 25' - bigger boat and sailing a bigger boat.
BUT - you said you're going to take lessons. I would take them while looking for a boat, not after you get one. You'll learn a lot about the boats, likes and dislikes, cockpit space, potential pitfalls of trailering and the things that will consternate a nice day sailing. Ultimately, I think if you take the sailing lessons first, you should do fine with the 25'.
BTW - just based on something you said earlier: if you're looking at sailing simply for fuel savings, bail out now. The folks who I've run into who've done it for that reason wound up not liking it. Too much work getting the boat out on the water. Sailing is MUCH more involvement with the boat moment by moment. You've got to want that kind of experience to enjoy it. I don't think that's what you're doing, but I thought I'd say it just in case.