It is entirely possible that Mr. Frazier was incorrect with his estimation.
That's possible, but how about observation? Frazier said Oswald carried that package in his cupped hand and tucked under his shoulder. If the package was 3ft or more, how tall must Oswald have been?
If the bag did contain curtain rods,
That's a very big "if" since nobody has ever seen them.
From a cupped hand to the tip of a shoulder could easily be 3ft. How much real attention did Mr. Frazier show and did he drop Oswald off or did they both walk from the car park to the TSBD.
Oswald had told Mr. Frazier the bag contained Curtain Rods. Oswald took them into work as he wasn't getting a lift home. That was the story he told Frazier, more or less. So what happened to the rods.
As to Mr. Frazier failing to verify the bag, he was shown under interrogation, as being the bag on the back seat of his car, that he briefly took a disinterested glance at. Well that's no big surprise as the bag would've been deconstructed by Oswald in removing whatever it was that was in it and had also been manhandled by numerous detectives.