I suppose what I'm really trying to establish is the location of the source of the photons we receive now in the COBE or WMAP telescopes. They would have been sufficiently distance to allow a travel time of 13.5 billion years but as the space between us and the source has been expanding I am assuming that they will have travelled from a distance which was much less than 13.5 billion light years at that time in history, just post big bang (after 350000 years, its all relative!).

Angular diameter distance is how far away the light source was when the light was emitted. You seem to be asking about the luminosity distance to objects whose light-travel distance is estimated at 13.5 Gly.

See Wikipedia: Distance measures (cosmology).

Luminosity distance can only be estimated by making assumptions about changes in the expansion rate over the last 13.5 billion years. I've heard estimates as high as 56 Gly.

Here are some graphs of various theories of how the expansion has evolved. Pick a theory and calculate an answer to your question.

For a grade-school simple calculation, you could assume that the Hubble parameter (expansion rate) has been the same as it is now, for the last 13.5 billion years. At the present rate of about 2.5 x 10

^{-18}/s, distances doubles about every 9 billion years. I worked that out with logarithms on Wolframalpha.com a couple of years ago. Just to double check, here's the problem in reverse:

9 Gy = 2.838 x 10

^{17} s

(1+H_{0})^2.838 x 10^{17 }= 2.033.

13.5 / 9 = 1.5; so the distance would have increased by a factor of 2

^{1.5} = 2.828. 13.5 x 2.828 = 38.18. Estimates higher than that, evidently, are assuming the the expansion, now, is slower than the average over that time period.