It's not clear to me from your screenshot how many channels you are actually trying to use concurrently. Is it just channel D0?
If so, then turning off the other active but unused channels (D1 thru D7) will possibly allow you to increase the size of the D0 trace. At least, that would be the case on the MSO5104 which I own.
If not, then you're probably stuck with what you have. The 5104 at least, doesn't allow you to position individual traces at will. The most you can do is resize them (screen real estate permitting vs active channels), or to reorder them.