It may well have been narrowed at some point but looking at this extract the rugby club is mentioned as being a tenant so I would say those current dimensions are probably correct.

The home pitch for Oldham RLFC at Boundary Park measures approximately 110 yards by 74 yards (101 meters by 68 meters). Because they share this traditional football stadium with Oldham Athletic A.F.C., the playing dimensions conform to standard association football sizing rather than a maximized rugby league layout.Pitch DimensionsLength: 110 yards (101 meters)Width: 74 yards (68 meters)Surface: Mixed-fibre hybrid pitch (upgraded with council investment)WikipediaBoundary Park - WikipediaField size. 110 yd × 74 yd (101 m × 68 m). Construction. Built, 1904. Opened, 1 August 1904. Renovated, 1991–1994, 2014. Tenants. Oldham Athletic A.F.C. (1906–p...