10.5 of course

yup. unless ur workflow really requires such size.

Else u may not be using it as a tablet but likely use like a laptop which u are likely to use it on table.

Yes, used to have a 12.9" iPad Pro but it was too large, too heavy for my usage. Especially one handed usage is tough

no prob.   

but eventually go with your own heart as you are the only one who really know what you need.

what's your ipad usage? 256 is a lot already

depends on what are you doing on the ipad pro i guess.

256gb to a tablet is considered a lot i would say.

of course if u can really make the ipad pro a replacement of laptop, then no harm to go for 512gb also.