vivian,

Its very easy task, copy the RNA sequence from file and paste into Microsoft word, then select Review option from menu bar, before clicking on  Word Count option select RNA sequence.to know length of RNA sequence. after clicking on Word Count option they display the total number of character present in sequence in box

try it.

By read length do you mean raw reads used in assembly or the contig length variation. From your contig file alone, you cannot determine the read length distribution, but you can determine the contig size variation.

By read length I mean raw reads -  how do I determine the length/size of the raw reads in RNA-seq especially if it is a large data?

Do you have access to any of the raw data? If so you could check a few lines manually or run it through QC software such as FastQC which will give you a graph of the distribution of lengths. This can be helpful if there are a variety of read lengths in your data.

How did you set your experiment? Did you sequence by yourself or did you use a external facility? In this case, did you chose a specific read length? There are many questions before being able to answer your request, as to know if there was any pre-processing of the raw reads before performing the de novo assembly (read trimming for example). How was the bioinformatic analysis performed and by whom? If you have access to the raw reads (fastQ file), you can run (or ask the facility for) fastQC which is a quality check software and will provide an analysis of read length distribution (among other useful things), as stated above by Cameron. If you have access only to your assembly and the contigs, then you must get access to the raw reads files...