I don't know how close the measurement needs to be, but I would eyeball the hubs at 1/4" thick, so that would make 87 inches.
Close enough to the 88 I measured... so 87 or 88 probably OK. Assuming they are in fact the same trailer (and one/both aren't slightly tweaked variants made for a custom order). 88 is a common straight axle size (as is 87), so either one is believable.
Edit- the keel roller setup is different... so clearly not the exact same trailer. As long as the springs are centered, I'm confident in my 88" measurement (69 + 9.5 + 9.5)
Spring mounts match... so that's not the issue.
Looks like my springs are the issue... which makes sense, at low load leaf springs shouldn't be nearly flat. Yours have a lot more curve... and the empty trailer's weight shouldn't matter that much. Heck, I stood on it and the only thing that compressed were the tires (likely underinflated, but inflated enough for running light). Also, yours has 4 leafs while mine have 3. Not up to task?
Per my measurements, I have a spring spacing of 69", and the spring length is 26" (frame) / 25 7/16" (spring eye to eye, no shackle). I'm fairly certain they're standard 1 3/4" wide springs... but I didn't get a measurement (and I'm out of town).
While I might be able to save a few bucks by having a shop add flanges to the axle, I'm thinking it might make more sense to just replace the axle and springs as a set. Around $600-$800 before freight from what I'm seeing. Not currently in budget, but this trailer is a long-term project, so that's fine if it's justifiable.