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First, some important things about portal length.

1) Portal length is important for keeping passenger cars from causing a short when the polarity is reversed for that end of the reverse section, and is the only thing controlling portal length.

2) Portal length is irrelevant for locos.

3) Portal length is irrelevant for non-power-pickup cars.

4) Lengthening, shortening, staggering, or any other modification to the portal will not help steam locos that don't have all-wheel power pickup to work.

5) Messing with the length and location of the portal rails will not do anything except for passenger car considerations as noted in 1) above. And messing with the configuration of the portal other than what's recommended here will most likely cause lighted passenger cars, or other power-pickup cars, to cause short circuits when crossing a portal when it's in the detection mode.

Hopefully, this makes it clear that there's nothing you can do with the portal to keep you from having to add power pickup to some steam locos, and nothing you can do to alleviate any other situation except for the one purpose the portal length is for - keeping lighted passenger cars, and other power-pickup cars, from causing a short circuit when the polarity is reversed at that end of the reverse section. So, when might that be?

If you have a passenger train that is seven feet long, but your reverse section is only four feet long, the loco will exit the other end of the reverse section before the train is completely within it. In this situation, the loco will switch the polarity to match the other end while passenger cars are crossing the first portal, with the reverse section polarity opposite that of the track on the other side of the portal.

The portal length being longer than the passenger car truck insures that the truck is fully within the portal before it starts crossing the gaps to exit the portal - thus not causing a short circuit.

The portal length being shorter than length of the shortest car (usually an old time 34' Overton passenger car), insures that the lead truck of one passenger car has fully exited the portal before the lead truck of the next passenger car starts entering the portal.

So, what does all this mean?

If you don't have three-axle power pickup trucks, the length can be shorter - just as long as it's longer than the longest power pickup truck you will ever have on your layout.

If you will never have cars with power pickup trucks on your layout, the portal can be as short as 1/2". But play it safe an make it at least 1" anyway. However, keep in mind that if you make your portal this short, you will either never be able to have power-pickup cars on your layout, or your reverse section will have to be long enough to contain the entire train that has power-pickup cars.

Yes, as long as your reverse section is long enough to contain the entire train, portal length is irrelevant. As stated before, the only thing that makes portal length important is to keep power-pickup cars from causing a short circuit when crossing the portal when it's in the detection mode (when the loco has switched polarity and power-pickup cars are still crossing the first portal).

As shown above, wheels (1) must clear gaps (2) before wheels (3) cross gaps (4). It doesn't matter if the front truck of tany car crosses the gaps when the rear trucks of any other car are also crossing, only that two front trucks or two rear trucks do not cross gaps at the same time.

If you have power-pickup cars shorter than 34' Overtons, your portals will need to be shorter. If you don't have anything as short as a 34' Overton, your portals can be longer.