https://www.ncbi.nlm.nih.gov/refseq/ will provide you a means of searching the length of CDS sequences that should provide you with the length of any cDNA you have synthesized if you know what gene you were specifically looking for. I'm not sure if that is what you are asking though. Without more information about what exactly you are looking at and trying to do with the information, it is difficult to answer this question.
Agree with Jim Ropa. Also, If you are looking for Arabidopsis thaliana cDNA references, you can get help from TAIR (https://www.arabidopsis.org/index.jsp).
Do you want to know how long your cDNA product is? You could run it out on an agarose gel next to a DNA ladder to get a good estimate. To know the exact size, you will have to sequence it from both ends and figure out the size of the paired consensus sequence.
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