Citrix documentation can be a bit hit and miss. I was struggling to find a decent reference architecture guide for XenDesktop 7.1 when I came across the Citrix Project Accelerator – https://project.citrix.com/. This is a basic design wizard for XenDesktop (and XenApp) which takes into account workload, user count, feature requirement etc. and then creates a basic design with bill of materials etc. This obviously isn’t capable of giving a full design, but as a starting point or sanity check of an existing design it does a reasonable job, and considering you can go through the wizard in 10-15 minutes it’s a worthwhile exercise.