Only picture I could find of my bench:
As someone who recently finished up a bench that I overthought and agonized over, I think the biggest thing is to just make a bench and see how it works. I used a coffee-table sized bench with a crappy vise for way too long; it was terrible and held my work back. I should have built something quick and cheap and better than what I had, rather than wait months until I could find the time to make something awesome. Mike Siemsen's Naked Woodworker bench or something similar would have gotten me a useful bench much sooner, with less bother and expense. With that, I could have had some real workholding while taking my time considering everything I needed/wanted in my omega bench.
That said, after nerding out for months and blowing my tool budget for the quarter, I ended up with something I really like using, and don't see replacing soon if ever. It's somewhat like Chris Schwarz's knockdown Holzappfel, with a leg vise, planing stop, holdfasts, a split top, and no end/tail vise. Having worked with it for a few months (exclusively hand tools, power tool work happens outside on sawhorses), here's a few thoughts from someone who was recently in a situation similar to yours:
Split top: love it. I've been lazy making the gap-stop piece, but when that's done I'll have absolutely no complaints. Also it saved me from joining two 3" slabs, and it'll be easier to manage two smaller slabs vs. one huge one when I have to pack this bench up and move it.
Leg vise: also awesome. I coughed up for a wood screw and a Benchcrafted crisscross, it's not quite as fast as a quality metal screw, but it's quick enough, holds like a beast, and looks the business.
Planing stop: I like it but if machines do most of your planing it could be foregone in favor of a couple pegs or a batten clamped at the end of the bench when needed.
Lack of end vise: I have no experience with an end vise, so I don't know if I'd like it, but I don't feel restricted by its absence. Holdfasts, the planing stop, and a notched batten work well and I get to avoid the extra expense and installation hassle.
On the subject of holdfasts: they're terrific, and I would prefer something other than the Phil Lowe bench just because the cabinet would get in their way. I got a pair of Gramercy's, highly recommend.
On using green slabs: Mine came from a Florence casualty, and were part of a log until last September. They're a bit cracked and still moving quite a bit, but it's honestly not that big a deal for most of my work. Basically when it's out enough to bother me, I plane off the high bits until it works again. Once it calms down I'll get the whole thing bowling-alley flat, but until then I'm not going to stress about it. Also, the holdfast holes go oval after a while, but a bit of work with a round rasp sorts that. One advantage is that wet oak is *heavy*. The top is probably pushing 200 pounds, and that mass is very nice. And it was cheap (traded for it), available, and I didn't have to find a way to plane and glue up a laminated top with no workbench or clamp collection.
Sizing: Mine is a bit under 6ft due to shop size (6x8 corner of the living room), and I wish I could have a couple extra feet. For height, the top edge lines up with my wrist joint, and I wouldn't want it too far off of that. If you won't be doing much hand planing or stock breakdown, and the bench will mostly be for joinery and assembly, you may want it a touch higher. For width, it's as wide as I can comfortably work on the far edge. About 24" I think.
Anyways, good luck and sorry for the text wall.