As you wrote : ... older ....
For us, the grand dad's, there was not many choices, 54mm or smaller . Then 75mm from Serie77 and the 75mm by Ray Lamb for Hinchliffe quickly followed by 90, 100, 120 etc ...
And certainly no bust, which I find an aberration
Remember, when you look at your work in 54mm on a shelve at 30cm, it's like looking a standing guy at 3m .
For bigger sizes, it means the guy is closer and closer so you see him with more details ( and less contrast ) that must be reproduced .
A good old 54mm ( 45 years ago, Humbrol ) Here 2 pictures for the same 54mm, 1st one you look a it at 30cm, the 2nd one at 5cm ( myopia allow to do that, referee in paints' contests are all myopic
)
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An 75mm same period ( Humbrol ), 1st picture, on the shelve at 30 cm, 2nd the way a referee look at it
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Conclusions : paint what you likes, the size you feel confortable with and if
you are not pleased enough with the result, scrap the paints and again
NOTE : you must know that you will encounter, one day, a piece that will fight against you and you will never be satisfied . It happened to a friend of mine with a landsknecht from Rose Miniatures, he painted it and repainted it, he asked me to repaint again, twice, no satisfaction, and again, he, me, he, me even the last try we made 40 years after the first one : NO WAY ( tried Humbrol, oils,acrylics, mix, pastels ... )
Go for it