Go bigger.....much bigger....if you think you will ever need it.  I put up a 60x22 ring 150,000 bushel Brock bin last year.  Dirt work, concrete, bin, erection, fans, electrical, withdrawal, and air system piping came out $2.48/bushel.  It was financed with FSA using a Farm Facility Storage Loan for 12 years.  I had three bids and they were are all similar, but the one I went with quoted everything correctly the first time.  When I did this it was the largest I could do in that price/bushel range.  If I went bigger things got more expensive, and if I went smaller it didn't get less expensive quickly.

I'll be the first to admit that putting a pencil to a project of that size can be intimidating.  Will I capture basis improvement in the market every year to make it work?  Probably not, especially since I don't raise enough corn every year to fill it up.  For me it came down to this.  My operation needed to raise more corn for several reasons.  Bean yields are/were struggling on a 50/50 rotation and corn yields are generally increasing faster.  Also, it allows me to harvest with a smaller crew and do it faster.  Finally, I decided I could raise more corn, take it to the elevator at harvest and give them the money, or I could build a bin.  I choose to build the bin and not have to plan my crop rotation around storage.