How can I work out what the aperture size is to replace this wooden sash window?

[MisterBoy]

I'm looking to replace this traditional wooden sash window with a modern (non sash) uPVC window:

I'm struggling to figure out what size the new window should be since the existing frame obscures it. The internal aperture is a different size to the external aperture, and the external aperture angles inwards. I obviously don't want to pull the window out before the new one arrives weeks later, which would be the easiest way to see how big the hole in the wall actually is.
I have a bit of familiarity with sash window - I know how to get the sashes out by removing the internal bead - but not enough to figure out with confidence what size the window should be. I assume we'd remove all the woodwork except perhaps the cill and maybe the internal lining if possible.
Is there an obvious trick to it or is there a bit of messing about to be done if I want to get a close fit?

Thanks.

 

[Deleted member 174758]

Either side of the visible window, hidden by the plasterwork reveals, are the sash weight boxes. This means you will have to break into the reveals in order to takevthe measurements. Thry have to come out in any case to get the old window out. Also fo the same for the head andcremove the window board to check the depth there, too

 

[MisterBoy]

So you'd break in from the inside of the room rather than remove the sashes and get in from the sides?

I never realised there WAS a plasterboard reveal but have just checked - thanks

 

[crank39]

On a sash box window everything on the inside is basically hollow and houses the cast iron weights, once both sliding sashes are removed and all the timber frame what you'll be left with is whats known as 'check reveals' google that term and see images and get an idea of what you'll have before ripping it all apart, but I usually measure outside brick to brick minus 10mm for the width, and height from the stone cill to the lowest point in the arched head, fit the window forward of the existing and in between the brickwork and then box the inside reveals out with 4x2, new window board, plasterboard screwed to the 4x2 for the reveals and the face just bond out and skim with multi finish

 

[MisterBoy]

On a sash box window everything on the inside is basically hollow and houses the cast iron weights, once both sliding sashes are removed and all the timber frame what you'll be left with is whats known as 'check reveals' google that term and see images and get an idea of what you'll have before ripping it all apart, but I usually measure outside brick to brick minus 10mm for the width, and height from the stone cill to the lowest point in the arched head, fit the window forward of the existing and in between the brickwork and then box the inside reveals out with 4x2, new window board, plasterboard screwed to the 4x2 for the reveals and the face just bond out and skim with multi finish

Really useful thanks. Measuring the brick to brick outside wall my first thought but as it tapers I was nervous in case the taper continues further back than it looks, on this old house anything is possible!

Sash windows I find really interesting and as long as I can put it back together I'm fairly happy ripping it open just don't want to end up with a big hole for weeks