Problems with doping semiconductor materials in 7nm technology
The presence of dopants does not limit itself to just the vicinity of those dopants. Its job is to add a discrete energy level that either gives an extra electron to the conduction band (donor) or accepts an electron from the valence band (acceptor) resulting in an extra hole. That extra electron or hole is free to move around, even if there aren't any dopants nearby. So not having any dopants in a \$7x7x7 nm^3\$ cube is unlikely to be an issue. 

Furthermore, a "smallest feature size of \$7nm\$" usually points to the minimum channel length of a transistor. Other structures may still require larger sizes.