I had a 'large' van that was going to have it's entire roof covered in solar panels, even then my sums said it only just have enough surface area. Electric power consumption is basically the same regardless of how big a car is, but a small car will have much less roof area.
The solar panels will also make the inside of your car much hotter. A Metro has space for about 150W of panels, this will give you about 60Wh max real world at midday on a sunny day in summer.
How efficient is the step up converter? It can be as low as 30% for cheap ones. If you want to run everything on the regulator, you'd only have 20Wh
The ignition might not be happy with less than 14v either. I was going to go to a 100Ah Life battery for a pretty decent amount of headroom (this would have held 14V for a long time).
If you spend a lot of time parked and commute a fairly short distance it can be done obviously, but you might be better off like a lot of people have done and just grid charge at night.