Ok, so I'll come back and add specifc quotes to this later to support each of these facts, but based on my re-reading of WoK;
The wooden bridges are no more than 30 feet long and 8 ft wide (WoK p.101), so that is the longest distance that would need to be crossed
Soulcasters have created multiple buildings that seem to be done out of thin air and not wood. I am assuming that they are not soulcast out of wood as they are usually described as being smooth and seamless, while soulcast wood to stone usually is described as retaining the wood grain. Barracks - Long enough and wide enough to hold 40 men and withstand a high storm. This has to be larger than a bridge
Hall of Maps - Large Domed structure that was soulcast. There is no indication that it was built of wood and then soulcast.
Several soulcast buildings are domes, which show the ability to work with arches and the strength to hold up a large amount of their own weight
This being the case, I would argue that the soulcasters would easily be able to cast an arched bridge, similar in size to a barracks, This would be wide enough to support a large cavalry charge. If large domed structures can be created that hold their own weight, then a span of 30ft or less should not be a problem.
As much as I would argue this, the earlier link to the Manchester signing and the WoB over rules any debate I might have on this subject.