Cast iron tubs are still pretty much the same size now, as then.

There are two dimensions, length and width, and found as a PDF format on the mfg. sites.

Length, width and height. This isn't cast iron, we're looking at porcelain-enamel over steel. I'm finding very few 30x60 tubs unless I'm willing to sacrifice height. Taller (standard) tubs with higher water levels are about 32". My concern (and need to post here) came from an online plumbing supply store that stated the following:
Product Specifications:

Overall Length: 60" (head-to-toe measurement when lying inside the tub)
Overall Width: 32" (shoulder-to-shoulder measurement when lying inside the tub)
Overall Height: 17-3/4" (exterior height from base structural support to top of tub lip)
Water Depth: 14" (height from tub floor drain to overflow outlet)
Maximum Water Capacity: 50-gallons.

The pdf led me to a completely different tub (a whirlpool tub).
"Overall" length is described as "head-to-toe measurement when lying inside the tub"...Really! :o They messed up.

"The pdf led me to"

To end up at a PDF, you have had to start somewhere, which has been omitted, thus far.

Standard castiron and prest-steel tubs are 30-32" wide, 60" long and 14-18" high, to the top of the flood rim.
No respectable plumbing sales person talks with reference to laying in or shoulder width.