ATMEGA16U-MUR is a VQFN-32 with exposed pad (At least according to digikey)
The packaging details are in the datasheet page 297.
The closest fitting one in the official lib would be
QFN-32-1EP_5x5mm_P0.5mm_EP3.8x3.8mm (Name as found in the v5 lib. This fp is not present in the v4 lib. The closest one in the v4 lib has an exposed pad of 3.45x3.45mm)
The one i give above has a slightly too large exposed pad. (I would guess better would be something between 3.6x3.6 to 3.75x3.75mm depending on your manufacturing tolerances and clearance needs.)
ATMEGA328P-PU would be a DIP 28 (Again digikey)
Part drawing is in the datasheet page 439
If there is a fitting part it is in Package_DIP (Housings_DIP in kicad 4)
DIP-28_W7.62mm should fit. (named the same in the v4 and v5 lib)
For details about how to arrive at footprint dimensions from datasheet drawings see: Tutorial: How to make a footprint in KiCad 5.1.x (From scratch)?