if you can't find information in the literature (or if you want to check if the length in the literature is the same that you would get working on a certain species), you can try to align the primer sequences to a reference sequence of your choice.
In order to do this you can download a reference sequence from a database (e.g. GenBank or BOLD) and open it in any alignment software (e.g. MEGA or Geneious) together with the primers sequences. Once aligned, you will see how far the two primers are..
According to Article Activation of Methanogenesis in Arid Biological Soil Crusts ...
Supplementary table 4 (https://journals.plos.org/plosone/article/file?type=supplementary&id=info:doi/10.1371/journal.pone.0020453.s009) the amplicon is 469 bp. They use a modified version of the forward primer, but it binds to the same site.
I think the issue fixed to me at the moment. For future, if I could not be able to find any paper, how to solve the Blast problem to have a strong and accurate document?