Resize Artboards in Photoshop (Multiple Methods)
Photoshop offers flexible methods to resize artboards depending on how you like to work. This tutorial covers resizing with the Artboard Tool, through the Properties panel, by dragging edges and corners, and how to resize multiple artboards together.

Step-by-step instructions
Create or open a document with artboards
File → New and enable Artboards before creating the document.
Confirm an artboard exists by its name label in the workspace.
Resize with the Artboard Tool via the Options bar
Select the Artboard Tool (nested with the Move Tool). Press V, then cycle if needed.
In the Options bar, choose a preset from the Size dropdown or enter custom Width and Height values.
Resize using the Properties panel
Open Window → Properties (the panel should show Artboard options).
Toggle the link icon to lock/unlock aspect ratio.
Enter Width and/or Height values, or select a preset from the Size dropdown.
Resize by dragging edges and corners
With the Artboard Tool active, hover over an edge until resize handles or plus indicators appear.
Drag horizontally or vertically to adjust the artboard size.
Drag from corners to resize in two dimensions (similar to Free Transform).
Resize with the Move Tool
Switch to the Move Tool (V).
Hover near artboard edges; handles are hidden but still usable.
Drag edges or corners to resize, similar to using the Artboard Tool.
Resize multiple artboards at once
In the Layers panel, Ctrl/Cmd-click to select multiple Artboard layers.
Activate the Artboard Tool.
Enter new Width/Height in the Options bar or Window → Properties to apply to all selected artboards.