Best to keep in the fridge, let the cake or slices sit out at room temperature for 30 minutes or so to warm up. Microwave for 10 seconds? Sure! Melts the buttercream a bit, but that isn't a bad thing either!
6" 2-layer cake feeds 4 people nicely, 8 people with an addition to the cake like fruit or gelato.
8" 3-layer cake feeds 12 people nicely, 24 people with an addition to the cake like fruit or gelato.