It depends on what you're requirements are (beyond the above). I say this because it does have a bearing. What type of seating also dictates this. Let's put this this way...

17" laptops do fit in J and some Y seats, however, in Y, they will often be digging into your stomach. Some 15" will as well. 14" works for me in Y. Just be careful where you put the screen (eg, do not put it under the tray table latch... assume the person in front will unexpectedly recline)

The bigger concern is the power requirements. Most 17" are gaming laptop to one degree or another. Most will require more than 100w power. The average power socket on a plane is usually around 90w tops (some will trigger at 80 or 75). If your flights are pretty long (eg TPAC or TATL or transcon) then that might want to factor into your pick.

I would probably aim for a 14" or 15" ultrabook (CPUs end with a u as their model number) unless you're doing something processor intensive. For most tasks (email/spreadsheets/general browsing) it should be enough (just don't have 40ish tabs open). Discrete GPU is unlikely. I find I can run 1-2 virtual machines based on a mid-level CPU (i5 or Ryzen 5) without too much of an issue, but I'm not thrashing something).

If you're looking for something off the shelf, Lenovo and HP often have sales (and you're in the right season for it). I find Staples sometimes has an odd good deal. Canada Computers rarely has a good deal (in terms of laptops) but when they do, it's pretty decent.