My heat just went off last night. I've been working with the furnace and the thermostat. Usually I figure out what's wrong. I think this time it's the fuse in the thermostat. I've misplaced my booklet. Can you tell me what type/size fuse I need. Thank you.

Don't believe there is a fuse in the thermostat. Look on the Transformer for the thermostat and/or on the control panel in the furnace.

It's best to look up your components by the model number, and then look for the manuals, or troubleshooting procedures. To my knowledge there is no fuse for a thermostat (Maybe you mean a transformer), but the accepted method of eliminating it as a problem is firing your furnace fan by connecting the thermostat wires. Did you look for error codes on your furnace?

Thermostat Home Furnace Troubleshooting-Step By Step Guide W/VIDEOS (http://preferredhomerepair.com/ThermostatTroubleshootingAndRepairGuide.html)

A Google search yielded many links to your thermostat, including this one

https://www.manualslib.com/manual/579102/Robertshaw-9500.html