Can porcelain tile planks be installed in a crawl space house?
I fully agree with Isherwood that it depends on the stability of the floor. I have attached tile directly to T&amp;G flooring but it was a bit tighter or more solid looking than yours looks like. I would put down backer board in your case I would trowel a layer of thin set with a large notch trowel put the backer board down and screw the backer to the flooring doing the entire floor with backer.  This will make your new floor +1/2” thicker but it will last a lifetime. If your floor is really solid you may be able to use larger tiles without backer and not have problems but I would caution not to use less than 2x the width of the existing boards to prevent problems with slightly uneven boards larger tiles are better in this case. After many years of putting tile down I almost always use at least a thin backer with the above method.