Log Cabin Floor

Hi All

I've recently constructed a 4m x 5m log cabin in the garden from Tuin. As it's a log cabin and it is contructed with the weight around the perimeter (rather than being built on top of a floor base like a shed) I thought I was being clever/saving money/time by building a perimeter base out of slabs. It did work quite well but I didn't really plan what to do for a floor.

My current plan is to frame the inside with 50x50mm (47mm) treated timber and then run 50x50 joists across the short length of the cabin (gaps filled with insulation board.) Obviously because the perimeter walls are on slabs they are higher than the internal base (see pictures on dropbox ) I know 50x50 isn't really a standard joist dimension but I am a bit limited by roof height and threshold height so my plan was to have another run of slabs across the centre to support the joists mid-way. This would give an unsupported length of 1.41m. I would then cover with 18mm OSB or plywood or similar.

So my questions:
Does anyone have any suggestion what the maximum unsupported length of a 50x50 joist would be,I'm guessing it may depend on width of joist centres?
If I was to use 2400 x 590 tongue and groove OSB would I have to put a noggin under the unsupported side?

If I'm approaching this all wrong and you have a better suggestion and/or want to berate me, I'm all ears!
https://www.dropbox.com/sh/1xnr2mod55hnb8a/AAAMOAUZOTv4U2rGJL6pgmjDa?dl=0
Thanks