I personally would be shocked if it's anything but 28mm heroic. I base this off two things, the first being that GW is a models company first and foremost, and the second being that the game will likely be targeted primarily not at WHFB players, but people who have come into the hobby more recently & TWWH players.
As GW is a models company, they have a lot of people who collect the models but don't play the game, and smaller scales like ~15mm hold a lot less appeal to this demographic due to the reduced detail. As far as the second issue, getting people to rebuy models is not a concern which warrants a scale shift because GW already knows that the old WHFB fanbase can't be relied on to sustain a major product line - else we'd still have WHFB today! They won't be primarily targeting people with existing WHFB armies, but rather newer players/people who never played WHFB (which will be a substantial % of the playerbase given churn/the outsize popularity of 40k) and trying to on-ramp TWWH players as well. Rebuying is not a concern for these people. While they will still want to attract WHFB grogs as well, this is where factions like Kislev and others (maybe Norsca?) come in, enticing WHFB grogs with a brand-new model line without requiring any kind of dramatic scale shift.
And all that aside, of course, the history of GW shows that people are often quite willing to rebuy a nicer version of what they already had! I'd certainly scramble to throw more money at GW for new Bretonnians if given the chance.