So, looks like the Starter Sets will come first, but after that first set, what sizes are you likely to need most?

The Auld Grump

I think for every 4 sheets of 6x6, I'd want one sheet of 3x6 and one sheet of 3x3.

Top-of-head estimate.

Sounds about right, maybe 6, 2, 1 in my case.

The Auld Grump, tabs... not so much....

That seems about right. Maybe a sheet of tabs to go with that. Actually, I might say two sheets of 3x6 to each sheet of 3x3 due to the number of tiles per sheet. I tend to use about equal amounts of 3x3 and 3x6.

In an average build I would say I use roughly:

20 of 6x6 tiles
4-6 of 3x6 tiles
5-8 of 3x3 tiles

Following roughly this ratio could be handy for starter sets and maybe even for figuring out how many of each in an a la carte fashion. To keep the number of sheets to a the lowest but still allowing for a decent 3'x3' build, maybe starter packs could be:

4 sheets of 2 of 6x6 tiles (a total of eight tiles)
1 sheet of 1 of 6x6 tile and 2 of 3x3 tiles
1 sheet of 4 of 3x6 tiles (a total of four tiles)
1 sheet of 35 tabs

That would mean only four layouts for the CnC machine to run while keeping the ratios of tiles close to what I would like to see. The thing is that is more sheets than I first set out to recommend and worry that it might possibly be too pricey for some to want to jump into. Maybe making only sheet type (1 of each with 4 tabs) is the answer. The ratio is off, but it might bring costs down to only run one template on the CnC machine. You could then sell starter packs with something like four sheets.

That sounds about the same as I am using at the moment. My builds are going rather slowly due to limited space to lay out the large sheets to cut down into manageable sizes.
I am just hoping that pre-made ones will not be too expensive to ship to England.

6x6 and tabs, but since that wasn't an option I voted 6x6 because they are to most time consuming.

So far in my builds I've used maybe a couple of dozen 6x6 tiles, half a dozen 3x6 and NO 3x3, so that makes my vote a fairly simple one

I build a lot of multilevel maps that make heavy use of 3"x6" tiles, so I have far more of those. Looks like I'm one of the few, though.

For me, it has to be the 6x6. There just isn't any other way to go. They lay down the base layer of whatever I'm doing. Multi-levels will get more into the other stuff, but the backbone will always be 6x6.