Personally if you don’t mind using an iPad as a laptop, rather than an iPad the 12.9 is fine, but if you want a tablet experience the 11” or mini is your best options.
I toyed with the idea of using an iPad as laptop replacement, but with the 12.9” I find most people have it in the magic keyboard case about 90% of the time including me, in which case you may aswell get a MacBook Air, which is not only cheaper but is far more capable and lighter.
Unless you really need a touch screen, Apple Pencil support and Face ID, I would consider that over the 12.9” iPad.
As for the 11” it’s a much better form factor however with a keyboard it becomes a bit cramped, but as a tablet it’s the ideal size, there is a reason why Apple have so many iPads in this form factor in the line up, for me however, even this size isn’t something I would carry around daily, your use case may differ though.
Another alternative would be the iPad mini if you want ultimate portability, this is a fantastic size if your going to be carrying it around on the daily and don’t need a huge screen for your use case.
So to answer your questions if the 12.9” is too big… it’s a case of it depends…
If you want it as a laptop replacement and your happy to sacrifice its capability for a miniLED touch screen and Face ID with the extra weight, then no it’s not too big.
If your going to be using it primarily as a tablet, go for the 11”, but if you really need ultimate portability get an iPad mini.