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

1.4 x 1.4 is pretty pokey , and no , I suspect an 820 door will be too wide unless you swing it outwards , which is just weird . If it swings inwards , even if it at 90 degrees to the toilet itself , there will be almost no room for a person , a WC and the door 'swing' . Most doors are at the end of the room , the WC at the other end , and that will make it even worse -- if the WC protrudes 50cm from the wall , and your door is 820 , you have 8cm in which to stand ! Setting the WC 'offcentre , the door off-centre the other way gives a bit more space , but add a basin and you have an obstacle course to negotiate . Is there any way you can take the length to around 1.8-2 metres ? A width of 1.2 works , 1.4 might sound better , but with only 1.4 of length you are pretty much stuffed . .

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.