I have a bare big-block that's been sitting for 20+ years (and wasn't pristine to begin with). There is surface rust especially on the bores, which are currently standard size. Normally I bring a block and new pistons to the machine shop and have them hot-tank, bore/hone to fit.

But in this case I'm not sure what oversize pistons to order (i.e. how deep are the pits) and if I guess too small, then I'm stuck with them.
Would the machinist be able to rough-bore until it cleans up, then let me know if .020, .030 etc. would be the proper size to buy?
thanks.

Why don't you hone it and inspect and measure? Otherwise, I would think you have them hone/bore to minimum necessary then buy pistons.

Two things I'd want to do are:

1) As Dragonslayer says, see if you can get an idea of the depth of the damage by honing.
2) I'd want to know how much wall there is, and how much core shift, via sonic measurement.

I'd get the block tumbled or blasted first. Once the block is really clean then you can take it to a shop and they can examine it and measure it and then go from there.

https://board.moparts.org/ubbthread.../engine-block-tumble-deburr-process.html

Personally I'd have a good shop clean and mag the block then rough bore it to figure the final bore size. Most good shops won't final hone the bores until they have pistons in hand to calculate piston to wall clearance. Remember odd bore size pistons can be spendy...i,e. 010,020 etc. 030 is common and easily attainable.

A very common problem. The only safe way is the right way. It costs a little more but it's the best. (Anybody that thinks they can guess the depth of rust is only fooling themselves.) First I will tell you how I do it. (Your man will probably not like my way.) And then I will tell you what he might prefer. But ask him how he wants to do it.

I hot tank the block and rough hone until all the rust is gone, then I know where I'm at and can shop pistons.

Here's what you will probably have to do. Find out which pistons you would like to use. Take the block to the shop and have him hot tank and bore to the size you want to use. If it doesn't clean up you will have to bore to the next size. Hopefully it is just surface rust and will clean up at your desired size. Then you can buy pistons and he will final hone.

Either way be sure you strip the block naked before you take it up there. Remove the freeze plugs, cam bearings (hopefully you have or can borrow a tool for that), distributor drive bushing, water drain plugs, everything. He has to charge you if it takes him an hour to strip it. Good luck with your build.

Let the machine shop clean up and inspect the block. They will let you know what size pistons you need.
They might even be able to source them at a better price?
Pretty much what they do every day.

If after honing the bore size isn't too big you may be able to get the existing pistons coated and use them.

https://www.line2linecoatings.com/