Unfortunately I do think you have to hold off for the moment, because we just don't have confirmed base sizes, other than old 20mm square bases are now supposed to be on 25mm squares. "most" 25mm square and 25mm x 50mm base units are also going to larger base sizes, but no specifics.
That info is from april this year -
link - which is new enough that it's unlikely to have changed, but old enough that it ~might~ have.
TBH I would hold off on prep work for a TOW army until we get finalized base size charts, but those might not arrive until the actual game does. I don't think official round base sizes for AoS were published until the game was released then either.
But if you must get a head start in on prep work, then the safest stuff right now is Skeleton Warriors, Skeleton Archers, and Tomb Guard, all of which should be on 25mm squares according to the info from April.
Infantry heroes - Tomb Kings & Princes, Liche Priest, Necrotect, herald - are also ~probably~ on 25mm squares to be able to rank up with the infantry units, and the new infantry kings we've seen look like they're on 25mm squares to me, but it's hard to say with new models, and I don't think we've seen any heroes actually joined up in units in any of the promotional pictures for TOW. Joining units might not be an actual mechanic in the game, in which case the base size of heroes might be different from their squads.
For those who want to avoid rebasing existing armies, or who want to maintain cross-game functionality with either Oldhammer or Age of Sigmar (but probably not both at the same time), spaced unit trays are an option worth considering.
LITKO sells laser-cut plywood trays that will convert both 20mm square to exactly 25mm square ("compatible with Oathmark") or 25mm round to ~roughly~ 25mm square (a bit extra rim space, as one would expect for a unit tray built for actual 25mm square bases). The 20mm square to 25mm square trays currently max out at 5x4, but I expect more sizes to become available after TOW releases, as there will likely be a demand for larger trays, particularly 5x5 and 6x5 or even 6x6 for hoardy anvils & chaffy tarpits, or wider than 5 frontage for offensive glass cannon units.
Coveted Forge sells 3d printed plastic trays, again both for
20mm square to 25mm square and
25mm round to 25mm square, customizeable with a wide variety of sizes and with divets for magnets if you're into that.
If you have a 3d printer it shouldn't be too hard to find files for printing your own unit tray spacers.
...
If you want to maintain compatibility with all 3 games at the same time, that'll be difficult, My initial thought would be to go baseless entirely, and pin some wire pegs out the bottom of your models' feet. You can then do different bases or unit trays for each game with holes drilled in to attach the pins to. This would be difficult with skeletons, but maybe not impossible. This or something like it might be the only viable option for AoS/ToW cross play for larger models like chariots and monsters, where you probably won't be able to fit the entire dimensions of their old world square base into their AoS circle or oval base or vice versa.
