Lol, thanks MAXXxxx. :smile:
I started out on the 28's (GW, ral partha, etc) and then 6 or so years ago started working on larger figures. These days I typically do 54mm and 75mm, with the occasional 28mm, 90mm, or bust. I think experimenting with different scales is a great way to push your skills and learn. You may find you prefer one scale over the others, but it's still worthwhile shaking things up every once in a while. By your questions it sounds like you're on the right track. I don't drastically change my approach when I work on a larger scale. Mostly it's about putting in more detail because you have more room to work. Faces are a good example. With the larger figures (be it 70mm or bust), you've got a lot more room so you can work on this more subtle lines and features. Or with eyes, for a 28mm I'm happy with I can do the whites of the eye and then a black dot for the iris/pupil. For 75mm I'm putting the pinks in the corners of the eye, using a color for the iris (blues and greens show up better then browns), a black dot for the pupil and a white dot for the catchlight. On my latest bust I'm doing all that 75mm stuff, throwing in shading on the whites of the eyes and on the iris color, I also tried to work in some blood vessels in the corners for the first time. You don't necessarily have to do all that (it took my many larger figures to get to where I am now), but the point is when you do a larger figure think about what details you would do on a smaller one and then think about what else you could add now that you have more room.
Yes, blending will definitely be important and these larger figures are a good place to practice. Mistakes in light placement can be more noticeable too. If you want, there are a number of tricks you can use to help with the shadow and highlight placement. On some earlier projects I would prime the figure (solid color), then place it under a light and take photos from the front, back, and sides. It's best to have the light above but slightly between you and the figure (whether it's facing forwards, backwards, or to the side). Those photos were a great reference, especially when doing the shadows and highlights on all of the folds in the clothing. Another option is to do the black/white priming (all black, the spray white from above). It's the same idea, just showing you what surfaces the light will hit. I don't bother with that anymore on the figures, but it was a nice way to start.
On the bigger pieces you can use oils very effectively, as Antar mentioned. But you can still do acrylics (I still use them on 75mm, 90mm and busts).
As for the relationship between scale and contrast, I'd say it's more about your style and preference than hard and fast rules. Technically, sure, at 1:1 you're using minimal contrast (though you can still do some, just look at the make up industry). In reality, I think you can do high contrast on a 75mm or a bust and it can look great. If you look at examples of busts and 75mm stuff on here or P&P and look at the highest rated ones, you will find they tend to be pushing the contrast even though they're larger pieces. Here's an example I posted on my blog a few months back showing a few different scales. You can see how the level of detail increases and how more subtle features and lines are brought on on the larger scales. But you should also notice that there's not a gradual decrease in contrast. If anything, the 150mm might have the highest contrast. I was experimenting with it and you may not like the style, but I do and don't think there's anything wrong with high contrast on a larger figure.
Here's another look after he'd had a bit more work done on him. Every shadow and highlight isn't black to white, but for the big shadows I get very dark and for the top highlights I'm still going close to white.
And here's an example from that 70mm figure for a closer look at the shading and highlighting. There's still quite a bit of contrast there.
Last bit of advice is remember that it's a learning process, so give it time and don't expect your first larger figure to be perfect.
