If you think you’d need an external monitor for something, I doubt if a 15” or 16” will be anywhere near a satisfactory replacement even temporarily. Of course, it’s (to me, uncomfortably) larger than the 13” or 14”.
I’d suggest physically handling those measurements before deciding. Two inches (and the added weight) can make a big difference in portability.
My preference is the 13” or 14”, mainly because I can use the price differential between that and a similar spec for the 15” or 16” (which could be a few hundred dollars) to have better internals on the 13” or 14” (like 32GB RAM and/or a 2 TB SSD). Since the RAM and internal SSD are not upgradeable on MacBooks (of the past few years and the current ones) after purchase, think about how long you would use the laptop before buying another one and what spec would work better for future proofing it.
Though the Apple Silicon Macs of the past year have been very efficient and performant even with 8GB RAM, there have been many reports of heavy use of the SSD (swap space for virtual memory), leading to suspicions/predictions that the internal SSDs may wear out sooner. I’d be a lot more comfortable having more RAM with these new systems and enjoying the performance without too much of a compromise in the long run. SSD wear is also inversely proportional to the free space available. So the larger the drive is or the less of it that’s occupied, the better.