Strictly speaking a "sector" is the physical unit the harddrive uses, usually consisting of 512 bytes and a "block" is the logical unit the file system uses consisting of a whole number of sectors.
However, many programs (and people) use "sector" and "block" as a synonym. This also applies to the HDF settings of UAE. Therefore "block size" should always be 512.
Reserved blocks is a file system thing, too and should always be 2.
Surfaces a.k.a. heads and blocks per track a.k.a. sectors [per track] should be choosen so that the number of cylinders does not exeed 65535.
The formula for HDF size is simple:
HDF size = cylinders * surfaces * blocks per track * block size
or the other way round:
cylinders = HDF size / block size / blocks per track / surfaces
(Win)UAE usually selects surfaces = 1 and blocks per track = 32. However your choice of 1GB just exeeds the upper limit for these values, so you should choose surfaces = 2 or something.
Be aware that the geometry is stored nowhere but in the UAE config file. If you want to use a HDF with a different config, you need to enter exactly the same geometry, otherwise the HDF will be unreadable. So always use 1 surface / 32 blocks where possible (and keep HDFs at 1023 MB or smaller).
About boot priority: the device with the highest priority is the boot device. The floppy drive has priority 5, so if you set your harddrive's prio higher than 5, you won't be able to boot from floppy disk any more.
The usual priority is 0. If there are multiple devices with the same priority, the one which comes first in the list (i.e. the config file) is used.
