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The usual approach seems to be that the end-block is about 1" thick and 5-6" wide square piece of mahogany etc. but does it really need to be this big? The more I think of it, does it need to be glued on the top and back at all? Does it really serve any other purpose than being a rigid point for the end pin? Wouldn't the guitar top be more free the vibrate if it wasn't glued to the end-block at all?

Yup, bevel the glue surfaces so it's no wider than the linings where it's glued to the plates. Its two purposes are as an an anchor for the end pin, and joining surface for the sides. Without the end pin, you could just use a thin offcut from the sides as a joining surface.

Plywood is great for this, I use 15 mm, 10-12 mm (1/2") is probably sufficient. Plywood is dimensionally stable, and unlike an un-laminated endblock with the grain running the same direction as the sides, it won't crack even if the endpin is wedged in too tightly. Definitely bevel towards top/bottom, if nothing else to avoid the unsightly "dimples" that can occur when the surrounding plate area moves with humidity.

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Rian Gitar og Mandolin

It doesn't need to be any more than 50mm (2") wide. Ply works fine, ~12mm thick. Bevel/scallop it down to the thickness of the linings for the top and back.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

I like a large tail block on my long scale bouzoukis to help with balance so it's not so top heavy. This probably isn't necessary on most guitars but my 10 string zouks are 27" scale, and with 10 tuners and a horn, there's a lot of weight that needs to be counterbalanced by making an appropriate sized tail block.

I think you are right. I have always built them the traditional way and it's annoying when you see the telegraphed dimple that the block produces. I've reduced the size of mine quite a bit though I still use mahogany or spruce. Building with a tail piece might require an adjustment to that but even a traditional Selmer guitar uses a smaller block then the traditional acoustic guitar as is drafted on so many plans and books.

Just adding to what Todd said...all the older Martin's I have looked at have a cross grain strip of spruce to sort of laminate and strengthen the tail block. I'm not sure if all vintage (or newer) Martin's have this, just the ones I have paid attention to. I suppose in theory this strengthens the tail block, but what about practically? Still see lots of older Martin's with end pin cracks. Maybe Todd, and others with repair experience on these could elaborate more. This is a pic of a 1932 0-18.

I think gluing the end block to the top and back adds a fair amount of crack resistance to the sides when the guitar is dropped on the end pin.
I usually use a piece of 1/2 inch multi-ply about 2 1/2 inches wide and the full height of the sides (glued to top and back)

Ditto on beveling it so that it matches the lining width top and back.
The endblock to my way of thinking does much more than hold the end pin (the classicals I make don't have an end pin).
I see it as an easy way to add mass to the back and side assembly without making heavier bracing or thicker plates.
More mass provides a better "inertia stop" to the action of the top and keeps the energy from being absorbed by a too light back and rim assembly.

"Light" is not the general rule in guitar making but "Right" is.