Wood changes most in dimension along the tangential direction with moisture change, typically 60% as much along the radial direction, and hardly at all lengthwise. (You can find lots of numbers for individual species in wood handbooks, but unless your source gives a variance (±) for each, they look more precise than they are.) Wood dimensional change is almost entirely due to the change in dimension of the wood fibers, rather than of the space between them, and is quite close to being proportional to wood density and moisture content:

dV = dMC rho

where
dV is the change in volume (within ±20% for most woods)
dMC is the change in moisture content
rho is the specific gravity (density) of the wood.

So, in adapting from 70% RH outdoors to 20% indoors, MC changes 10%, from 15% to 5%, and a wood of density 0.45 shrinks about 10% of 0.45, 4.5% in volume. The change in dimension tangentially will be about 64% of the volume change - 2.9%, the radial change 36% of the volume change - 1.6% (again, ±20% for most woods).