When deciding which windows to choose, you must first and foremost ensure you satisfy any requirements stipulated when receiving planning permission and ensure you satisfy local and national Building Regulations.
There are ratios to consider and minimum overall U-value parameters you will have to abide to, which your window retailer should be aware of. Check for yourself the technical guidance documents (ROI) and technical booklets (NI). The ratio of wall space to windows needs to be carefully checked and balanced to ensure you encapsulate the views and light but also balance this with energy efficiency as set out in the building regulations. For more on this topic see this article comparing double versus triple glazing.
Thereafter you’ll need to consider the following: