I have been looking over land in the local area. As much as I am happy living, and working an acre, with access to a few more if necessary, any sizable shelter will need a bit more space to work it properly.
If you aren't forced to evacuate underground, then the more space you have, the more food you can grow. It's more to work, but more people you can support. There are many things worth considering - does the land have a creek or at least some sort of water, for example?
If you have to evacuate underground, then there are whole new considerations. Are you going to build horizontally, vertically, or both? Vertically takes a lot of very good design, takes up less ground area, with much higher costs; horizontally makes any building works more obvious, is easier to do, takes up more ground area... less shielding against radiation. A combination has some of the benefits, some of the problems.
Of course, a workforce who can do this (or at least arrange this to be done) makes the task easier... Of course, you have to do things in a way that local government won't notice, or worry about, or that curious people won't poke there nose into, but...
Prices of around $1 million for a few hundred acres are high, but would be worthwhile investment for a group... Even if an investor was to front most of that in order to ensure their survival, accepting the face that they need support.