New Build - is this A/C sufficient

House is a single storey 290sqm (31sq) with 2.6m ceilings.

Builder initially quoted us for a Fujitsu 10KW ducted system. After questioning if this was sufficient they advised that it was but i could upgrade to a 12.5KW or 14KW if i wanted.

Even at 14KW i think that is still way under the mark for this size of house but i'm not really basing this off anything other than my opinion.

We could take the A/C out of our build but it would obviously cost us a lot more money to install our own post handover. But also dont want to end up with something that struggles in the western Sydney heat and ends up pointless.

It all depends on how well your house is insulated. For standard BASIX 6 star build it is certainly not enough.
But for 10 star it may be even over excessive.

We selected to upgrade from the BASIX insulation to R2.0 wall and R4.1 ceiling. Which also included an Enviroseal wall wrap to external walls.

There isn't a mention of the star rating of the upgraded insulation though so not sure where this lands.

how many and how big west facing windows

The back of the house is south west facing which will include 1 bedroom window and the rear stacker doors between the living area and alfresco. Alresco is 4.5m deep so shouldn't get too much direct sunlight until later in the afternoon. There are a few smaller north west facing windows which will get sun all day though (in the living area these are around 2000 (h) x 800 (w).