In terms of external dimensions the Kestrel and Merlin series are actually pretty close, the big difference is in cubic
displacement and weight.
Kestrel(unsupercharged):
Length 66.72"; width 24.4"; height 39.4"; weight 840 lbs.
Displacement 1,296 cu in.
Kestrel(supercharged):
Length 69.82"; width 24.4"; height 37.53"; weight 950 - 970 lbs.
Displacement 1,296 cu in.
Merlin (single-speed supercharger):
Length 69"; width 29.8"; height 41.2"; weight 1,375 lbs.
Displacement 1,637 cu in.
Merlin (two-speed supercharger):
Length 71"; width 29.8"; height 43.0"; weight 1,450 lbs.
Displacement 1,637 cu in.
Merlin (two-speed, two-stage supercharger):
Length 88.7"; width 30.7"; height 40.0"; weight 1,550 - 1,750 lbs.
Displacement 1,637 cu in.
All figures are from British Piston Aero-engines and Their Aircraft by Alec Lumsden,
and reflect the basic dimensions of each family. The actual dimensions, particularly
length, varied by specific engine sub-type.