Apologies if this is lengthly, but I've gone back and forth on this for years so figured I'd share my thoughts. Of course, all of the usual caveats exists - depends on your workflow, what you want to do with the device, everyone is difference, etc. etc.
TL/DR - The 11" iPad Pro is the safer bet for an all-around usable device. With display scaling you can get almost to the same usable space as the 13" (but it's small). The 11" iPad is a tablet that can scale up to a so-so laptop if needed, the 13" is a so-so laptop that can scale down to a so-so tablet. That being said, you seem to really want to experience the 13" which is why I think you should get that one and use it for a bit. I'm normally not a fan of arbitrary returns, but if you don't get the 13" you will always be wondering if you should have gotten the 13" so you need to experience it.
Expanded Answer: Historically I always had the smaller Pro models (the 9.7" and 10.5"). With the M1 I wanted the new screen and bought into the entire iPad productive narrative and went all in with the Magic Keyboard and Pencil. I even affixed my 10.5" to construction paper for a week leading up to pre-orders to approximate using a device of that size - ha! The 12.9" iPad Pro was an impressive device and for the first month or so I used it extensively, however I noticed that over time the size (not just weight - which has been improved - but the overall dimensions) made it a bit awkward for more casual things - sitting in the floor with my rabbits, reading a book on the cough, etc. Again, some people swear by it, but I found it cumbersome. I even bought a small metal stand from Amazon which worked great, but then I was lugging around an iPad and a metal stand everywhere which was lackluster.
Fast forward several months and I ended up getting a 6th generation iPad mini at a steep discount. I decided to pick it up to be an eReader since the 12.9" was so clumsy with that. What I found over the next 6 months was that without even thinking I began using my iPhone or iPad mini in nearly all instances - even when it wasn't optimal. I'd rather reference material on my phone then get up and lug in the large iPad. The 12.9" became a glorified TV for eating and treadmill use which felt awful given the price.
In May 2024 I decided to consolidate the mini and Pro into a single 11" M4 iPad Pro expecting it to the best of both worlds. And in many ways it was, but as
@MacDevil7334 points out - the 11" is actually a bit of an awkward size. I definitely reach for it more often than I did with my larger Pro, but it was still kind of big to just grab and go or toss round. Reading books and manga was
fine - but it was still a bit fatiguing after a wile and with manga I felt like one-page portrait was too big, but two-page landscape was a tad small. I also got into classic handheld emulation with Detla and I find the iPad mini is perfect form factors. Large enough to have 3x screen scaling but still have room for controls and old Pokemon games and related ROM hacks are easy to play without a controller. These are a bit large and awkward on the 11" which also isn't very comfortable to hold up in your hands for long periods of time.
All this to say, that for non-productivity traditional personal computer stuff I find that I gravitate towards an iPad mini, or at most 11" iPad Pro, because I want something light and easy to pick up. If I could only have a single iPad it would be the 11" iPad Pro because the hardware is so much better, but if the mini gets the rumored sizable updates I could see the Pro line just getting priced out of my interest unless my artistic hobbies ever grow sizable enough where I could maybe see getting a wifi only home-only larger iPad Pro in many years - but it would be a niche device and not a general personal computer.
As for productivity, meh - this goes by the person. Personally I find even a 13" screen cramped. You can maybe have two usable split screen windows, but the experience is not night-and-day better than the 11". Even with iPadOS 26 I just find productivity on iPad awkward - like I'm using the device with mittens. Window controls and menu bars hide on hover, winding just feels clumsy and app scaling is inconsistent. Honestly the more they push the Mac-like multitasking the less I enjoy it. I feel like the old style was fine for my light usage.