Measure from the inside of your doorjambs. Be careful to note how wide your doors can open, and accommodate for doorknobs. If the doors can be removed, that might be a good idea to prevent damages. Always try and choose the doors that might offer the most generous space.
Measure the width of your stairway (taking account if you have a handrail or post), and the height from the top and bottom steps to the ceiling. Be sure to look out for any sloped or curved surfaces. If your stairwell has a landing, make sure your piece can fit on and around it, both width and height.
Account for any low-hanging ceiling lamps or fans, breakable decorations that cannot be removed, stairway banisters or turns, or ceilings and walls. Be sure to completely clear a path for your piece; it’s always helpful to make more space than you think you’ll need, if possible.