You can say a lot of bad things about Ikea, but not that thier products are impractical or not well thought through. After hesitation they started selling the Sladda here, but the price was lowered by a 100 euro's in comparison to other countries, and is now within 2 months after introduction 399 euro's, and the trailer is 159. 'Here' is in the Netherlands, a cycling country with a bike lane to Ikea's front door and a cargo bike rental service at the Ikea.

The Dutch have a bit of reputation among manufacturers for complaining about the quality after a bike couldn't take all of their abuse. Ikea has a reputation to loose so the fact that did eventually put it on the market here tells me that is very likely Ikea is confident the Sladda products are well made. Just not to the degree that they could ask 699 or 499 euro's for the bike, but if it was really flimsy or poor durability stuff it would be a PR-disaster to sell them here at all. So chances are it's a decent product.

Make sure it fits, but the manual can be downloaded and shows how to connect it to your bike.

Won't a close estimate not be enough? You've got the measurements, would it be that hard to estimate the width of the tyres, fenders and tubing and calculate what you have left for the payload?