Use Mr. Malty to determine how many ounces/ml of slurry needed for your batch.
As far as the yeast washing goes, I’ve read a lot that emphasizes washing but personally I’ve never washed my reused yeast and have never had a batch under attenuate, get contaminated, or result in less than desirable beer. The most I’ve reused is to the 3rd generation. If you have the time and means I’d wash but if you don’t you’re probably still going to end up ok.
My process for reusing: I only reuse yeast used to ferment beer less than ~1.055. After racking a batch off the cake I’ll split the yeast cake into 2 sanitized vacuum sealed bags then store in the fridge. For my next batch I determine pitching rate based on Mr. Malty and either pitch straight into the wort or make a simple starter then pitch.