Easiest way...
open your steam library
right click on gal; civ 3
click on properties
click the "set launch button"
In the box type: NoMinSpec
launch game
you should now have access to all map sizes
Beware however, you will quite likely run into performance issues, you have been warned 
When they first started limiting access to different map sizes, we raised a bit of a ruckus, so they made this workaround for us.
TBH it seems to me that they were overly restrictive. I routinely play map sizes 2 sizes above the restriction without issue, however your mileage may vary
Enjoy!
Oh yes... the launch option will remain until/unless you change it, so you don't have to keep messing with it.
