how are you able to pick and choose
 

how is it possible for people to say on here that they only solve problems of a certain percentage. i do not get any puzzle statistics and so when i press start the puzzle it is just pot luck whether it is a 4x4 or a 5x5.

Before you click "START PUZZLE" there is a "This Puzzles Statistics" box that shows the successful completion rate. 4x4 puzzles have generally a higher completetion rate (60% or better) while 5x5 puzzles are lower. So, I simply hit refresh until I get a low number. I've seen <20% for 5x5 but they seem to hover around 45% to 52% on average.