You can draw your own stairs instead of using the stencil from the building Core.
How to draw Stairs while drawing floorplan
Select Line and draw evenly. After adding enough lines, group it together, so the lines will not get messed up.
How to draw Stairs while drawing floorplan

And, you can change the size of the stairs by dragging. You can draw two walls and insert the stairs in between them. You can make your own stencils and add it into the More shapes manually. So, you don’t have to create one each time you draw the plan.

Microsoft Visio 2016 Step by Step Book by Scott A. Helmers

Perfect book for the basic use of MS Visio. Helps to learn more about the functions in Visio and also simple drawings.

It is a great How to guide for visio basics