What do you want in the ensuite? Double vanity? Double shower? Bath? If so, what kind of bath? A separate / private toilet? I think the answers to those questions will direct you to what shape room will best meet your requirements. But, in general, a long, narrow design seems to be the most appropriate, with the shower at one end, sink/s in the middle and toilet at the other end.

If you search DIY banquette on Pintrest you’ll see instructions and measurements. Most are in inches but you can convert them. Could you measure the seat height of the chairs you are using with the table (you need the table first) subtract the cushion height and that will give you the bench height. It does depend on the height and weight of those that will be using it the most, to get the best fit.

if you open the door in the middle back against the basin at one end and toilet at the other the minimum space to to consider would be about 90cm x 190cm and use the smallest toilet and basin depths but would be better to post the whole floor plan to see all the options

You have an ensuite and an identical bathroom around 3 steps further apart. Can't see any advantage for this. You could lose the ens, relocate the WIR and end up with a bigger bedroom, bigger WIR and save money. Your roof has these eaves all close together over what appears to be some upper level windows, but these walls aren't reflected below. If I've understood this correctly you've added significant build cost for little gain. I think the room sizes are ok but I would suggest a design professional to polish it up.