Think that there will be an arm available for that at a reasonable price or still too large?
It's going to be a combination of the weight and size. The 55" is just under 20kg (minus stand), so the 48" will be less, closer to 15kg probably... there are certainly arms which will take that (easily), but the issue would be the size, given how large the screen is. There would be an arm which could handle it, but it would need to have some height itself in order for the screen to clear the desk. If the VESA mounting holes are like the current range, they're in the bottom area, so that does help given there's less of the screen below where you'd be mounting it... and that's the concern for clearance from the desk.
The other issue is if that would end up pushing it too close to you... again, this is where a low profile wall mount comes in handy, as that's going to ensure it's as far from your eyes as possible, making it easier to adjust your seating position.
Is a wall mount not an option for you? Otherwise, I think you'd find some arms to take the weight, but it might be a bit of trial and error finding one with adequate clearance. I wouldn't rule it out entirely, and there's a healthy selection of candidates on the rainforest.