In 2022, the Welsh Government announced plans to increase the size of the Welsh Parliament/Senedd Cymru, a culmination of two decades of elite-level argument over whether more members should parallel the institution’s growing suite of legislative and fiscal powers. While an expanded legislature may improve the Senedd’s capacity to scrutinise the executive, opposition has focused on increased costs that detract from core public spending. Using a novel survey experiment conducted in the 2021 Welsh Election Study, we assess public attitudes and find that support for expanding the Senedd is limited to those with a strong Welsh-only identity and pro-autonomy preferences. Although appeals to holding the executive to account do not broaden this base of support, a framing that the Senedd should be comparable in size to legislatures in Scotland and Northern Ireland does have a positive effect on voters with more median constitutional preferences.