If you choose 6" size either the height or width will be 6" depending on the longest side of the design. All decal designs are different and most are either wider or taller. For example if you want an 8?.
eBay
Kids Loft Bed Frame with Slide & Ladder, Pine Wood, 97x208 cm - Space-Saving Children's Furniture
Aliexpress UK
Manual/electric Telescopic Loft Ladder Electric Model Retractable Hidden Staircase
Aliexpress UK