if you asked what a small project is I wouldn't be able to give you a confident answer
Anything that can be completed within 1 year :P
I tried to give myself a project that would take 1 month, in reality it took about 4 to 5 months.
Another thing about length of time working... I have found that sometimes I get more done in 15-30 minutes when I know I have to do something directly after, than in 2 hours. But sometimes I would miss that opportunity because I would think "I can't get anything done in 15 minutes." Not true! You can get 15 minutes of work done... and since it is a small time investment, even if you don't get much done it's not a big loss and you won't be as burnt out.
Of course, if you are really needing rest then that is more important. But if you feel inspired and only have a short time frame, you might be surprised what comes out when you really focus for a short period.