I recommend 40 amp - that is what my Fiesta (2018) uses. Square (Insert) Cartridge type
If you have the older 2011 Fiesta - that has that "wedge-shaped" fuse box in the engine bay.
R7 spot is the one for that 40A relay.
Surprised that the local Auto Store didn't know about the AMPERAGE of the fuse the Fan is powered with.
Perhaps it is due to the fact that the Fuse covers a BUSS supply of several devices, so if the Fuse quits - others sections that are supplied power by it - also fail. Don't worry, you're safe - the F3 40A fuse is specifically for that R7 relay. That Relay is rated 40A - so when you pull it off and find you have to replace it with a 3-rd party marketer - off-brand stuff - make sure that it is rated for 40A or above - no less.
Why do I ask, well a thread some time ago talked about the capacity of different Relays and how some relays have different size pins that can only fit in specific locations and only if it's oriented a certain way.
This thread might help you...
https://www.fordfiesta.org/threads/overheating-issue.8441/post-21986
You asked about the Serpentine belt, that would be a 6PK1030F.
I'm also going to let you know that AC systems in cold weather conditions will not kick the fan on immediately - for it can cause another condition of the engine not warming up well enough to handle emissions when it's too cold outside. Until the motor and the ECM agree it's warmed up enough - that fan will not kick on.
How do you know?
Look at your Tach on the Pods - your dashboard.
When a cars warmed up and in Park - that idles about ~700-750 RPM.
Turn on the AC - you should see a rise in the pointer and then settles down to about ~800 RPM.
But when it's cold outside - that temp sender it telling the system "What? Are you Nuts?" when you first turn on the Defrost. Because the system is looking at two things, Warm up - and outside temp - it's not conducive to run a fan to blow on a cold motor - takes longer for the engine to warm up so it "waits" to turn on that fan until the engine is hot enough to stay hot while the AC works and the Fan can blow on the motor and not suck all the heat out of it.
So until RPM drops to that "idle" point - the system is not in a "lean mode" - but it might be in a LEARN mode instead.
But that does not mean the
Compressor is not kicking in, - that's where you take a look at your dash to find that out.
Look at the RPM needle - if its' above your typical "idle" speed - then the compressor is on or at least the system is idle-up mode.
To prove that turn on the Climate control to Defrost and 1 or 2 on the Blower setting - with Heat on.
You may hear the compressor "kick in" and RPM jump as the system adjusts for load on the engine. The Fan might not. But, did RPM Go up?
Ok, turn off the Climate (Back to O setting) and wait one minute - this stabilizes the gas and pressures in the AC system the Pressure sensor verifies that the gas pressure is within range as a means to validate the emissions and load idle-up process. The RPM gauge will "stay up" and then relax back to idle in park speed - if it does this - then the system is ok - it's just too cold to turn on the FAN due to temperature and remember that pressure of a given volume of gas is different as temperatures change - so this is a simple self-check the AC system does as it provides a health-check for next time you use it.