Length of Pz 38(t) was 4.54 m/ 4.61 m. Length of Hetzer was 4.87 m and with gun 6.8 m!It was longer, because unlike the Marder series the gun overhung the chassis. The superstructure and tracks were widened, not the basic chassis. Plus of course all the alterations were IOTL done in 6 months, from concept to production.
Width of Pz 38(t) was 2.12 m, Marder III 2.16 m and Hetzer 2.63 m. So it was definitely wider.
Also when looking at picture of Marder Ausf M and Hetzer in Czechoslovak Armored Fighting Vehicles 1918-1948, page 225 it is clearly visible Hetzer is wider. Tex on page 227 clearly states
The width increased from 2.14 m of the PzKpfw 38(t) to 2.63 m
I would say slight increase in the length could be explained but different design and shape of the front of the hull. Also larger wheels were used so maybe bit there.