To start building the level I created a cube, 10cm by 10cm (the same size as I designed the platform here: https://jamesmwake.wordpress.com/2015/09/10/refining-level-designs-using-adobe-illustrator-photoshop/).
Secondly, I imported my previously created level props (e.g. platforms) and scaled it to 10% of its size, 1cm by 1cm.
Once scaled, the platform sat comfortably within the room.
From here, I repeated the process until the level was complete, using the cubes as building blocks. Adding any extra details at the end.
I used this method as it ensured everything stayed in scale, also providing me a quick and efficient way of building level designs.