
Hi there!
You should come and have a look round the 'Viva Strings' forum - lots of us stringy-folk hang around there.
I'm an adult who began violin just over a year and a bit ago. I bought a cheap 4/4 before finding a teacher who then told me my instrument was quite 'clunky' and heavy built although it sounded allright. My hands are quite small and it had a very thick neck so I was finding placing my fourth finger on the the lower two strings almost impossible.
I knew a 3/4 would be far too small so I started looking around for a 7/8 - there are not a lot of them about! My teacher had one but although I could play it much easier than the 4/4 it almost put me off because it was such a horrible sounding instrument. Then I had a 4/4 and a 7/8 sent to me to try. These were Romanian made violins and not hugely expensive. The 7/8 was again definitely easier for me to play; I could stretch across the strings and my fourth finger didn't hurt so much. But, my teacher did tell me my fourth finger would stretch with time and it probably hurt because I hadn't used it like this before.
However, I thought the 7/8 had a sweet sound, not quite as deep as the 4/4, but very pretty anyway so I bought it. It isn't hugely smaller than a 4/4 but just has a 'daintier' sized fingerboard. They became popular in the Victorian time as 'Ladies' violins' and some people collect them(!) just for the size. You still use a full-size bow and at a quick glance you can hardly tell the difference.
But over the last 11 months my fingers have become much more flexible and agile and I'm beginning to find my intonation is better on my father's 4/4 so I have finally decided to put my 7/8 up for sale (it's on ebay just now) and buy myself a 4/4 after all. I'm sure you could find a 7/8 in this country to try, so avoid shipping from the US, it might be a cheaper way of trying an instrument and you could always test it for size. Try looking on ebay or definitely at www.elidatrading.co.uk - their cheaper instruments are great quality for the price and perhaps they'll have something second-hand - worth emailing them to ask.
I don't regret buying the 7/8 but I am surprised how much my fingers can stretch now and how comfortable a 4/4 feels. A bit of me wishes I'd bought the 4/4 at the start so I wouldn't be changing instrument again right now (I now have to try to find myself a decent 4/4!).
Everybody reassured me that the 7/8 wouldn't sound 'smaller' than a 4/4 but to be perfectly honest, all the 7/8s I did try definitely did sound like a 'smaller' instrument with a thinner sound, although sweet. I played around with different strings too to build up the sound but it's still quite light-weight. It's probably a good ensemble instrument but a bit insubstantial for solo work - mine did get a bit lost playing beside a piano.
As a beginner myself I'm not qualified to give advice but IMHO I would go for a 4/4 if you can for tonal quality and strength. If you can have one made for you, you could ask for a slimmer neck like my teacher did with her custom-built viola. A 7/8 might make life easier for your fingers for a while but you may find if you work hard your fingers will strengthen and become flexible enough to manage ... but then again I'm six inches taller than you so probably have bigger hands. Maybe it depends on how long your fingers are; mine are quite short and my neighbour who is your height has longer fingers than me! I doubt very much that the difference between a 7/8 and a 4/4 would contribute to your back/shoulder problem; it's maybe something postural your new teacher can sort out for you.
Good luck - come to the 'Viva Strings' and let us know how you got on.