Draw SVG opens with a blank document the size by default. You can draw in this document or create a new one with the desired dimensions.
Open the "Document" menu by clicking its title, then click on the item "New".
The dialog panel "SVG document dimensions" opens and you can specify the document dimensions.
The panel allows you to define the dimensions of the document display and its coordinate system.
The "Full window" option allows you to define a document whose size occupies the entire display of the document window. Click "ok" to create the document.
This figure shows the two dimensions of the document
Then you can draw basic graphical items "Draw elements" or predefined shapes "Draw shapes".