What size PC do I need for Blue Iris?

I would always suggest you get the fastest CPU and GPU you can afford with enough hard disk space to save about a week's worth of 24/7 video (you should always record 24/7). To figure the HD space is not that hard. Just use this tool. It will also tell you how much bandwidth you would need to get to the NVR or cloud storage.

You should also look at Optimizing Blue Iris's CPU Usage to get the most out of the hardware you have.

But if you are trying to sort the minimum CPU and GPU you can get away with or trying to justify the cost of an upgrade I should note it is almost impossible to get a definitive improvement number of X setup over Y unless you have both running side my side looking at the same cams. Things like the amount of motion the cams are seeing, overlays and other features in use factor in too. However this page Blue Iris Performance Statistics has a sortable and filterable table of config and load data collected from around 1400 user servers. But that still just gets you in the ballpark.

For instance I have this i7-4790 I bought used almost 5 years ago now with 10 outdoor cams  that was running 50-60% CPU when I looked at it the other day when it was relatively calm. Today it looks like this, maxing out about 72%

Iris2 load and config

I built a i7-8700 with those same 10 cams to replace the 4790 PC. Even after adding another 6MP cam it was running 20-30% CPU during the same period as the above. Today it is maxing about 21%

Iris6 load and config

While both have the same configs for the cams in common but even without the extra cam that is not definitive for the CPU because the newer machine also has a better GPU (730 vs 1050), twice the RAM and is running BI5 instead of 4. Though the newer system actually appears to be using the GPU where the older one does not.  (The 730 does not appear to be supported for much.) Below are my other 3 servers to give an idea how motion, CPU and GPU factor in. All use an SSD for the Blue Iris DB and close to the same HDs for recording so the storage should not be much of a factor between setups. Note you can look at the motion column in the cam listings to get an idea of the amount of motion each cam relative to others on that server is seeing though the numbers are not relative between servers / consoles since the counts all start at server reboot. Also in almost all cases other than cam 93 the ones showing NoSignal counts are WiFi cams.

This one is close to max with 9 outdoor and 3 indoor cams on it.

Iris3 load and config

This one pretty maxed at 15 outdoor cams on it. Note I noticed the next day, after rebooting for updates, it was using the GPU pretty heavy. Similar CPU load though.

Iris5 load and config

And just to show the diff motion can make, this one with 23 active and 1 offline indoor cams that could handle a few more.

Iris4 load and config.

Update time to shuffle cams again. Having 27 indoor cams pushing almost 1000 MP/s is maxing Iris4