Hi karinasomers,
Welcome to Affinity Forums :)
Go to menu File ▸ Document Setup..., insert the new dimensions you want in the Dimensions field(s), make sure you have Anchor to Page enabled in the Objects will section and select the Anchor postiion you wand to use then press OK. If you are using artboards, select the Artboard you want to change the size in the Layers panel, switch to the move Tool and drag the bounding box handles on the work area or use the Transform panel to define specific dimensions for the selected artboard.
