Where are you putting the towel rack?
Above the heating vent is ideal but this may be too close to the toilet.
Are you changing the shape of the window? Or replacing in the same size/shape?

I was thinking towel rack at the other end of the shower on the wall. It will be a glass panel walk-in shower. I guess kind of like this style (though not exactly and not in black):

I was thinking to just keep the same size/shape of the window, but I'm open to suggestions if people think bigger/smaller would suit better?

It would be great if you could go a highlight window with towel rack over the vent but it might interfere with toilet as suggested.

Just on a practical level , going to a window that is bigger in any dimension ( wider or taller ) probably will be too challenging / expensive , as you'll need to change the framing ( often double framed around windows ) , the external materials ( imagine the hassles with brick or wooden walls -- not so bad if it is rendered sheeting or similar , but by the age of the interior , I'd suspect probably not ) , possibly even supports .

Personally , I'd stick to the same size window as it seems a logical fit -- maybe a modern black frame , or white gloss painted wood sort of similar to Kates pic ( 1 pane or 4 , fixed or opening ? ) .

Something like this??

On a practical level it works , just looks a bit plain to me -- almost why bother changing from the existing one ? Yes , 1 is aluminium , one powder coated , but I'd go chunkier or contrasting or something that isn't too out there , but isn't similar to what you have -- or stick to what you have .

With almost everything else changed , I suspect the existing window would look good .

I actually found and really like this design too, please let me know your thoughts?? Could work with a white or monument window. front of our house has monument windows

Also, I could have the towel rack like this maybe and keep the sliding door?

Would love some more opinions???

Use the same size and type of window . If you are not keen on the black metal , then just do the window in white . The position of the towel rail is good but I would put a small one under the window as well .

I could do a louvre window with privacy glass - you’ll get more airflow than with an awning window

New house near me has black window and white plantation shutters inside. Looks great, noting the window frame does not cross the shutters.

For the window, as long as it can open for air flow! Would prefer with fly screen as we get a lot of mosquitoes at night in our area!

another inspo pic

Personally I prefer the one with the bath and the shutters (would solve the window problem).

You could paint the metal Monument, remove the timber frame, tile the wall and into the painted metal frame, I would also want a bind for night time use otherwise it looks and is cold. I have just seen your ducted floor vent?, so I don't think you could have an open shower running across the end of the bath room?