Scaling images, whether from a preliminary sketch or a reference image, is almost never as straightforward as it seems. Here are a few things I think about:
By scaling up “mechanically” i.e. measuring or gridding, the overall geometry should of course work out, however that’s just the start!
We seem to scale up an overall picture space and then make elements within the space smaller than they should be. Let’s say something is a quarter of the size of the picture space – do make sure this element is a fully sized quarter in the larger format.
With reference photos, cropping for paint-suitability and size-appropriateness is invariably a good idea. Many photos, even professionally taken photos, do not necessarily make the best compositions in paint.
An image itself has a natural scale: decide what that may be or change the image – perhaps re-study (a fresh composition sketch!) to interpret on the new scale better;
Something will be lost – the almost abstracted first look at a subject, perhaps – so something must be added – maybe a graphic design element or emphasis. This is what the Group of Seven and Tom Thomson did, their full size paintings being much more “Art Nouveau” than the sketches. I talk about this in relation to Tom Tomson’s Jack Pine in this video segment: https://www.youtube.com/watch?v=yyBs7hNLxRo&t=10s
You can’t just scale the brush – brushwork is going to be different. Although using a bigger brush is one thing, you also need the brush to move in new ways;
Colours that may look saturated on a small scale may wash out on a larger format – more colour perhaps!
Similarly, line and value work differently with scale: the effect of scale on line work particularly is often underestimated, resulting in somewhat “spindly-looking” line effect;
What you can’t see or don’t notice may be glaring on a larger scale, an empty space or discontinuity for example;
All-in-all, there are many things to evaluate and perhaps consider additional studio techniques that might work: wet-on-wet, impasto, rollering, palette knife etc – although not all at the same time!
Break-a-Brush!
David