Layers and their borders/travel possibilities

In a purely geometric sense it is of course possible for an object with a defined shape to have a center even if it is infinite in scope (a parabola is an obvious example).

So if you consider the Outlands ring a perfect circle, then yes the Spire being infinitely equidistant from all points on the edge, could be considered the 'center' of the plane. However, on the Outer Planes physical measurements of distance only matter as a concept of short-term travel, head out towards about anything further than a day's march away and the rules of distance break down completely.

For example: if a Modron walks from two points 100 kilometers apart on the Outlands, an infinite number of times, taking the exact same size step each time, it records an infinite number of different travel times (though they might vary only be rather small amounts)

Of course, on the Inner Planes the rules are different, belief doesn't bend reality in the way it does on the outer planes, and physical distance does matter. Of course these creates a paradox that the Inner Planes are supposedly infinite in size, and yet you can walk from one to the other, but it's probably best not to think about that too hard.

I have my own pet theory on the subject.

So the fluff is as follows: most planes are infinite, but the setting seems not to be affected by the infinity; the way between two places (say, gate-towns) seems to change and, at least in the Outlands, geography doesn’t follow the laws of geometry; travel times further change depending on circumstances: doing Evil in Elysium means you’ll never get anywhere, for the good guys everything is just around the corner.
Explanation: every infinite plane is actually a Schrödinger planar pathway that doesn’t actually exist physically until observed, at which point it assumes a form based on who the traveler himself is (especially his deeds alignment-wise), where he is heading and the context of currently existing nearby terrain. Any actual place – town, divine realm – is a demiplane connected to the pathway (except the rules for interdimentional travel treat them as one plane).

Also, don't you think it'd be neat if doing Evil in the Outlands would pull you closer to the Lower Planes' gate-towns?

Physically, each layer of each plane is infinite. This includes the layers of Baator, the Outlands, and Bytopia, all of which have some perceptual boundary.

In Baator's case, the layers seem to get smaller. Dis is smaller than Avernus, and Minauros is smaller still. This is either (A) an illusion fostered by the baatezu, or (B) a perception of the visitors to Baator, as comprehending an infinite expanse of evil may be unsettling to some, or (C) blatant misrepresentations of Baator. It is likely that the baatezu promote this limited-size concept to deceive tanar'ri, or to get visitors to arrive in the center of baatezu power.

Bytopia's layers are also infinite. However, they are also bounded on one side - basically, where Shurrock and Dothion meet. However, both layers are infinitely wide.

The Gray Waste has all three layers meeting in a single central city. Similarly, Graz'zt's three layers of the Abyss also co-mingle. I assume the layers still have some form of 'reality boundary'. This might be similar to the city of Cynosure in Grimjack, where you could change dimensions by crossing a street.

In most cases, the depictions of various planes has been simplified for planars and the clueless. For instance, Gehenna's four layers are sometimes thought of as four sides of a pyramid. This is simply a concept used to form a mental image of the plane. Perhaps it is easier to navigate toward Gehenna if you imagine a pyramid.

The Outlands is a special case. It does have a real center (the spire), though the properties of distance are funky. These properties degrade even more the farther one travels from the Spire - in the Hinterlands beyond the gate towns, distance really breaks down.

As far as travel between the layers, this depends on the edition of D&D you use. In the first edition, it was possible to travel from layer to layer using the spell Rope Trick Furthermore, there were frequent portals between the first layers of adjacent planes, so you could travel from Arborea to Ysgard by travelling through a portal (the Per frequently guard these). In second edition, the Rope Trick passage was largely dropped, in favor of the various Planescape travel methods. In 3rd edition, the methods of inter-layer (and inter-plane) travel has been largely omitted, to my knowledge. Portals would have to be rather rare (otherwise the Gatecrasher prestige class gets REALLY disgusting)