You'd have to edit the 'default' xpack which contains the cross section.
Find your BTB installation folder, then the XPacks folder.
(e.g. C:\Program Files (x86)\
BobsTrackBuilder\XPacks\Default.zip, or C:\Program Files (x86)\Steam\steamapps\common...)
Make a backup of this zip file
Extract it to a folder called Default, then go back to the BTB installation folder and put that new folder in there. It should look like this:
Run XPacker (in admin mode) and you should be able to select the 'default' xpack. Find and edit the track cross section. It's best to
first remove the grass material from each side,
then delete the outer points from the cross section (in that order). Remember to click the Save button.
Go to File and Zip to BTB. If you've made a backup of the original default.zip you can overwrite it.
Final step is to copy the zip file into your project's folder, In there you will see an XPacks folder, which has a copy of the old default.zip. Overwrite it with the new one.
If you do it right, every new track will just be plain tarmac, 10m wide.