I dont shoot a 7mm so I cant expand a neck and give you an exact number, but I can get you close, assuming you're using brass that is similar neck thickness as mine.
My 6.5-284 Lapua brass is averaging about .015" thick. Necking up will probably thin them by about .0005, I say this because when downsizing from 308 to 277, I gain .001 and when downsizing from 6.5 to 6mm I gain about 1/2 thou.
Lets just assume for a minute that your expanded necks will average ~.0145"
That's .029" of neck, plus the bullet of .284 should give a loaded round of .313" on the neck diameter....................I would order a .312, .311, .310, and .309 to start with. Use whichever one gives the best measurements and least runout once you've got everything in hand. But still tight enough that bullets don't move under recoil or when chambering the round.
Also remember, that not all bullets will be exactly .2840" diameter. Some brands may be .0005 bigger or smaller. I've seen this in many other calibers, don't have any reason to think 7mm would be any different. That's just one more reason to order more bushings then you think you'll need, just in case.
Also, if someday you decide to use Norma brass, it may or may not be the same neck thickness as Lapua. Or if you decide down the road to turn the high spots off the necks, the .309 will be tight enough to still give good tension.
Hope this helps.
