My first go at making a CNC router

[JAZZCNC]

The x axis will be around 100cm and the gantry (y axis) will be around 150cm. Is this considered small?

Not small but still within acceptable range of ballscrews without taking special measures.

The L shaped gantry in your photo looks to be using two lengths of 80x120 making the gantry 200mm high. Is this right?

Yes ITEM 80x120

My original plan was to make a router that could be upgraded in size (4'x8') when the time came. That's why i was going for R&P and not ball screws. Is this a practical approach considering that the alternative would mean starting from scratch?

In theory all sounds great but in practice theres that much work and setup goes into machine to start with that you'll probably not want the hassle of converting.
Personaly I'd build with the intent of building another machine when want larger. You'll then be able to put all you've learnt and design changes into the new improved larger machine. Use the First to build second then Sell it complete and working. If do this right you'll make money rather than costing money converting.