Transform dialog

Scale tab of the Transform dialog (Object → Transform… (Shift+Ctrl+M)).

Mouse

With the handle of the bounding box, see Inkscape - Bounding Box (Selection cue)

Key

When an object(s) is selected

• < and >: Default scales by 2 px
• Shift + < and >, you scale by 20 px
• Ctrl + < and >: scale up or down to 200% or 50% of the original, respectively.
• Alt + < and >: scale by 1 pixel at the current zoom (i.e. by 1 screen pixel)

Offsetting

Inkscape - Path Offsetting (Inset/Outset)