According to the wiki the Resistance uses old refitted MC80s including Home One so technically we already have some Resistance ships. Also the Resurgent class star Destroyer is only 600m longer than an ISD which is 40% larger not twice the size.

ISD: 1600m

Resurgent: 2916m (to nearest m)

Not twice the length - but pretty close

Ah my mistake, last I looked (a long time ago) the wiki quoted it as 2200m rather than 2900m ish. Even so with a sliding scale it only need to be a few inches longer to give the impression of a larger ship. It will still dwarf MC80s which are the only known Resistance capital ships.