How to increase the size of value of X/Y axes in Simulink scope?
Unfortunately Simulink scopes have very limited formatting available. You have already accessed the styles that are exposed (plot background is one...)

There is two options

1) send to workspace toolbox and then use the plot command and edit the figure...

2) In the Simulink scope: File -> Print to figure 

Depending on the exact manipulation you are after #2 is generally quicker if you are after a quick format update. Once the scope is in a figure you can goto: Edit -> Copy Options -> Figure Copy Template ... From here the "presentations" default (and apply to figure) and usually suitable for a quick report/presentation insertion.

The correct and repeatable method is #1 as you can then create an m-file specifically for your formatting needs