Ok, depends -
There was an Original model attached. There were 2 other files where that original was either Inserted or copied at a larger size.
Changing the size of the Original is / was done with Modify > Scale tool. Can be uniform or non uniform.
Simple change to do with a little math. (Zero engineering)
The other Dave inserted the model, and probably scaled it at the time of insert, as I can't find that operation in his timeline. As said he then reverse engineered it, by copying the model into Fusion sketches with Projections - (purple articles) and associated features.
Then you have taken his sketches and copied those as new (mostly blue sketch articles).
Now sort of what you started with, making this model from scratch, is easily done, but I see a lot of easier ways to do it, than you have done it. To make it changeable with parameters, will take some better preparation. Changing dimensions - needs dimensions in the sketches, there were none.
Due to the organic nature of the floor surface definition, it is easy to replicate it with a surface body.
A solid Loft will need more profiles but can be done. Just a different order of operations.
How did the original modeler make his a solid body and not a surface body. Usually, Surface bodies are used in a construction / preceding mode and are used for the shaping of the end result - a solid body.
Some Modify Tools are used when appropriate.
So in a nutshell, is best to take a good look at the model, break it down into manageable steps to construct it.
This one has a Slot shaped extruded wall, and a filled floor cavity with wavy top.
You would need Length, Height, Width, and Wall thickness as parameters, so use those to get started.
Am I just missing some key steps to make the curvature solid, just one more step,
Extrude to Object, the appropriate floor sketch profile, and select the surface body as the extruded height. Hide the surface body, and making sure it was a Join, will have one body to cut in Manufacture.
Mine is not your size, change parameters to bring it to your size.
Might help....