I believe that the concept of a Cloud is really something that exists publically and is open or representative of the Internet, which by definition is a Network of Networks.  Private Exchanges for commerce are growing and can be distinguished as separate from Public Exchanges and would not be represented as Part of the Public cloud but rather the Private Cloud.  So the definition of a Government Cloud and Private Cloud that I get is that both are Private Networks similar to Intranet Infrastructures of Enterprises.  I don't think I can answer this question well but I can at least think about what is involved in answering it. 

How many Network Devices and Servers are associated today with the Public Cloud? Does include servers holding information put in place by companies/Enterprises and the Government that are open to all to access? I think it should and should be large in terms of the "Public Cloud."

How many Network Devices and Servers are associated today with the Private Cloud (Enterprise) whose access requires Secure Access Entry or Intranet/VPN access for their services - A Private Cloud?

How many Network Devices and Servers are associated today with the Government's Access Entry or Intranet/VPN accessible service or the Private Government Cloud?

One could also look at these via Bandwidth instead of Devices but this would vary from Network to Network and be hard to estimate?

I think from the above that it would be possible to compute the Existing size for each Cloud based on device numbers.

Projecting Growth is another issue:

My Guess would be that if the above definitions are acceptable than the Public Cloud would grow the fastest in my opinion.  Why, because we know that in High Tech Marketing the most important effect to get at $Value for the Internet is the "Network Effect.”  The more users who can connect to the potential solution the higher the value for Internet Site which will drive the building of the Cloud devices.  While a secure sign on is needed to get you onto a public site that does not make it a private site.  The Public Cloud serves sites such as Facebook et al.  From this, you would expect the growth of the Public Cloud to outgrow all others as the money pumped in from Venture Capital to Internet Ventures and Enterprises would be looking to get that Network effect going and grow out the Public Cloud. 

The Federal government is essentially broke since they mismanaged the country's finances, and they are cutting funds to the States so the state’s growth in Government Cloud would be minimal since they are being squeezed. 

Both the Federal Government and US State Governments, on the other hand, would need to try to cut cost and they both may have to put everything on-line and automate services to reduce cost via a Government Cloud to cover their deficit's.  The Federal Government will probably not be focused enough to make this happen quickly over the next four years and with the economic recession, so growth will be slow for the Government Cloud in my opinion.  (Oh I forgot being broke never slowed Government spending- my bad.)

I'm not sure how to look at the Enterprise Private Cloud. If my definition is reasonable than you would find that most companies would be trying to push their investment into technically what is defined as the Privately owned but Public Infrastructure to sell their products and services. For the Private Networks Enterprises would try to maintain and make more efficient their existing Private Clouds. 

This analysis is not very good since I have little facts and have not secured the definitions but I hope to learn more about the Cloud definitions and see if I can learn more about how others define and measure these.
Mark