I'm reading To Hell and Back by Ian Kershaw and he mentions that of the 1966 members of the 1934 party congress Stalin had 1108 of them arrested and had a total quota for the NKVD at one point of arresting 250,000 people. It struck me that this may have had a negative effect on the success of communism later on in the post Stalin era. All of the best and brightest communists as well as engineers and intelligentsia were dead.
Was the scale large enough for this effect or was it mostly just Stalin's personal wrangling in of the party?
Alec Nove argued in An Economic History of the USSR that the failures of the second five year plan (targets had to be revised down as they were not going to be achieveable etc.) was probably related to the purges. However there were probably a lot of contributors to that failing like inadequecies of development in the first plan's implementation etc. so it would be hard to say exactly how much of an effect the purges had on their own.
On your second question about the scale and bredth of the purges - the purges were not just limited to the party. For example there were a number of 'mass' and 'national' operations where the general citizenry were arrested for nationalist deviation, past political affliations and even things like vagrancy. J. Arch Getty gets into the institutional politics (with regional bosses possibly driving the mass operations, rather than the centre) behind the mass operations in The Road to Terror. There would have also been an intensification of purges in areas that had previously been subject to scrutiny such as industrial managers, who were frequently party members.