Realistic Spending Level?

What is the realistic level of federal spending in percent of GDP terms? A key question as we seek a long run budget stability. Matt Miller in WaPo worries that Erskine Bowles has short circuited the process by giving an unrealistically low level, lower than even that proposed by Ronald Reagan budgets (which had the baby boomers paying into social security and medicare, not taking out). It will be very interesting to see what the Fiscal Commission proposes, and further to see how much of it the President decides to put forth in his budget next year.