Using in a new design: Vishay's PowerPAK 1212-8. Tiny, tough, easy-to-cool.
At 3.6 x 3.6mm footprint, it's a fraction the size of a high-power SO-8, but sports similar-class die-carrying capacity, Rds(on), theta(ja), etc.
theta(j->solder_point) ~= 2.4K/W, as compared to 1.8K/W for the much-larger power SO-8 variants with big metal pads, and 16 K/W for a BCP53/BCX53 (random choice) SOT-89/SOT-223.
Package footprint theta(j->s.p.)
-------- ------------ -------------- SOT-223 7.3 x 6.7 mm 16 K/W SOT-89 4.6 x 4.25mm 16 K/W PP1212-8 3.6 x 3.6 mm 2.4 K/W PP SO-8 6.2 x 5.2 mm 1.8 K/W
AN822 starting at page 8 of this datasheet[1] has a good discussion about this package AND empirical SMD heatsinking considerations in general, with spreading and graphs.
[1]The PowerPAK 1212-8's thermal performance is excellent. For this design, it was a game-changer. SOT-89, R.I.P.
Cheers, James Arthur