It pretty much depends on what you are planning to do. Nevertheless, you may want to go through this paper.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3481852/

To me serum volume seems OK, check Ali suggestion

it depends on your isolation protocol. Generally 200 microliter serum is suitable for RNA or miRNA (small RNA)  isolation. Also, you may want to increase your start volume of  serume, if you have low quantity RNA end of the isolation. In this situation, you have to increase your lysis buffer, wash buffer and other buffer volume according to your serum volume.

from our data it is ok for miRNAs

Thank you for all comments. The main problem of my project is the old collected specimens, which may affect yield and integrity of RNA. I will try doing it. 

Another good source to look at might be: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3498755/

This publication does bring up the concerns you currently have.

When in doubt, TRIzol is always a good option: in your case perhaps 500 uL of serum would be added to 5 mL of TRIzol and you would proceed with the usual procedure from there. Add molecular grade glycogen (to the 2.75 mL of recovered water layer) to assist in eventually visualizing RNA pellet (and aiding some in RNA precipitation).  At the point of resolubilizing the pellet, add 50 uL of nuclease-free water back to the pellet. At this point, you have concentrated the RNA ~10-fold and should have a good opportunity to get an A260nm reading - and perhaps enough as well to use for e.g., Bioanalyzer 2100 RIN analysis.  I have had 100% success with TRIzol on numerous sample types. Hopefully your samples have been stored safely in the meantime.  It appears you are looking for mRNA and not miRNA - and so, you will be dealing with small amounts of total RNA to begin with (in serum/plasma).