Cannot read a thing. Elevations of the home and what the view is also figure into these decisions.

On the left and make sure they also stack on the left side, so they should open right to left.

I can’t seem to login properly to respond to your comments. Apologies for the low res, I couldn’t upload the files, just screenshots of them. extension is 3m and overall space is 6.8m wide internally.
I have considered equal sized glazing to left and right but it seemed more expensive to get two sliders, also, for one very large slider in the centre it’s pretty pricey and i feel like the space might be quite untethered. I don’t want to have wall space immediately next to the kitchen as I want to see out to the garden. I’m a landscape designer so hoping that all the views will be good eventually!

No glass will be least costly. If that exterior space will be your primary cavas maybe larger but fixed would reduce the total.

I am not sure what the issue is center the post and do two sliders then lots of options .Both sliders open from the center where the post is .

As you enter the room you'd have wall and not window. That is how it currently is and its not a great view. I think I thought teh table might sit in the middle some of the time but actually it is probably more likely to sit to the left or right, so a central pier would work. I thought that one larger slider (3.5-3.8m ish) would create a better view rather than 2 smaller ones. 2 sets of sliders is quite pricey too. I'm tempted though to do 1 set of sliders that are as doors (kitchen side) and another set as actual windows (living room side) about 40-50cm off the ground so you have a usable cill and it feels more contained

Also, by having a wider slider, I could have 3 panels and get a bigger opening. I wonder if having 2 sets, I'd be annoyed by that piece of wall in the middle breaking up the view? Its not that wide an elevation, 6.8m internally, so minus about 60cm each side we're looking at just over 5.5mwide minus any pier in the middle

Structurally a header transverses the window opening sized to support the load above and void anything in the center. 5.5./18 ft.

Climate and sun come into consideration. West facing will require shade and South can need a Summer overhang. Reflexive 366 soft coating for the interior of one pane of a double glazed insulated unit can help performance. You can choose one segment as a swing opening glass door. A wider center fixed and a fixed other side the width of the door side.

With dynamic landscaping windows to the floor will flow the view and feeling out to your work.

There's new tech if you have a cold climate at times. Vaxuum insulated glass is better insulation than triple pane.

" Vaxuum insulated glass is better insulation than triple pane. "

At what cost? Vacuum insulated glass is super expensive still now today and very few actually offer it. While great theoretically it really is still not ready for primetime.

The OP is not in the US.

Panasonic Glanevir is available through 2400 trained installers with government subsidies to upgrade single pane common in Japan. Panasonic uses precision machinery idled when they discontinued plasma tv production. This allows flatter tempered glass production. Better for the higher performing soft 366 radiant reflective coating. They offer annealed versions for windows with no sun exposure at a lower cost. They are actively ;incensing their tech and partnering to produce vig in Europe.

Know the price then decide your priorities.