Assuming you are using the most recent version of blast (and you should), you can ask to add the query (or hit) length to be added in certain output formats, such as the tabular one.

you'll need to add the following to your blast cmdline:

-outfmt "6 std qlen slen"

this will add, to the normal output the query input seq length and the hit seq length in the second-to-last and last column respectively.

See the blast help (blastp -help) for more info on those parameters (and how to add others for instance)