This is the alternative channel selection screen you are showing.
On this alternative screen you can change the size of the fonts on the left, but the font on the right might be problematic as it is bringing in all them times via my custom OnyxNextEvents2 python component. (it might work).
The screen is
.\usr\share\enigma2\slyk-onyx\
o-channel-selection.xml
<screen name="SlimChannelSelection" ....
If you open up o-channel-selection.xml in an xml editor like xml wrench (free), you will see I have titled comments for each section. So you should easily be able to find the elements to edit. For example
<!-- now programme -->
<widget source="ServiceEvent" render="Label" position="20,21" size="281,20" font="Bold;18" foregroundColor="colour2" backgroundColor="#25000000" transparent="1">
<convert type="EventName">NowName</convert>
</widget>
<widget source="ServiceEvent" render="RunningText" options="movetype=running,startpoint=0,direction=top,steptime=80,repeat=0,always=0,oneshot=0,startdelay=6000,wrap"
position="20,48" size="281,173" font="Regular;16" foregroundColor="white" backgroundColor="#25000000" transparent="1">
<convert type="EventName">FullDescription</convert>
</widget>
The panel on the right is this section.
<widget source="ServiceEvent" render="OnyxNextEvents2" position="1000,220" size="240,500" font="Regular;14" start="0" lines="26" foregroundColor="white" backgroundColor="#25000000" noWrap="1" transparent="1">
<convert type="ServiceName">Reference</convert>
</widget>
If you change the right panel, I don't know whether or not you can change the number lines or not. I would change the font size first, then experiment with the number of lines to see if it causes any problems. I never wrote this component, its someones elses code I amended for my skin. So I don't know how it fully works and I cant remember if I hard coded in the number of lines, or whether that lines variable does anything or not.