Copper/wort boiler question

[Cully]

I have my Brewwzeebo. I have my 40 ltr Buffalo HLT. I have my Xtreem coolbox mashtun. Time to buy my copper. I was going to buy a 60 ltr, 2 element plastic boiler but notice that the supplier also does 30 ltr ones. I had decided on a 60 ltr as I thought that there would be no chance of any boilover. Now I am wondering if this is overkill and whether a 30l one would work just as well. I plan to do mainly 17ltr brewlengths with the occasional 23 ltr one thrown in. So what I am asking is this; Is a 30 ltr copper going to be ok for 27 ltrs of wort? ( I am working these figures from a sample recipe that calls for 35 ltrs in the HLT to end up with 27 ltrs boiling down to 23 in the copper). Or am I better of going for the 60 ltr job?

[sbond10]

Always best to go slightly bigger and as most use 40 litre buffalo jobs I reckon your best with 60.