Blue tongue skink

[Cheesenugget]

I'm going with a t5 because it will be about 17" off the ground in the enclosure so about 14" of the top of the substrate, I'm building which is more then the t8 is rated for. I'm one of those no heat device without a thermostat people. So will use one for my own peace. But thank you all for your advice. I've still got a bit of learning to do. Keep it coming at me I love constructive criticism.
The enclosure will be 48"X24"X17.5". Was planning to use an RHP for heat source.

They like to bask in light so a basking bulb is preferred. Where will you place the UVB fixture? If it will be inside, a T5 is still too strong. T5 are meant for desert species and to penetrate the top mesh lids in most glass tanks where uv rays are 70% filtered through. Too strong of UVB light can burn your lizard, especially the eyes. Also, your bluey may refuse to come out and bask if the uv rays are too strong. A T8 is suitable for inside installation with a Reptisun 5.0 or 10.0.

The enclosure size is fine. The 24" width is great for them because they like to make large U turns so their tails won't hit the walls.