Your project is not large it is a good size for your first job. I use the 10 wire cause it does not cost much more and it gives a cushion. I like to overbuild things a bit
Remember to put conduits at bed/lawn intersections and anywhere you think will be prone to wire damge. If you use well lights of any sort make sure you surround them with pea gravel for drainage and to keep the heat down.
As far as what tap to use first it is kind of a educated guess at first try. If you have a longer run with multiple fixtures on it you can guess you will need one of the larger taps. The 12 tap is ussually only going to be good for a fixture say within 45' of the trans. There are alot of variables that attribute to voltage drops- coiling excess wire attributes alot to this.
Sometimes I think it would be easier to use 120 on everything and sometimes we do for that very reason.