Depending on your etching setup, 0.3mm may be too small, and you might find you're etching away part of your tracks or you might find shorts between them. Home-etched boards generally require a lot more space to prevent these problems, I would probably shoot for around 0.5mm minimum trace width/spacing.
For my professionally made boards I generally aim for 20 mil (~0.5mm) minimum distance between any copper or pads and the board edge. For hand-etched boards I would probably aim for about twice that (~1mm) minimum from the board edge. This will minimize the chances of problems.
It does very much rely on your experience and confidence level, though. I suggest giving it plenty of space to begin with, and as you make more boards and you find the tolerance is pretty tight, you can reduce the clearance and trace widths as needed. It's entirely up to you, but the numbers I offered should at least provide a jumping off point.