in my opinion, the use of electroejaculation in dog is not necessary and little bit drastic. Very good expert in obtaining of the semen of dogs is Dr. Prinosilova form Veterinary Research Institute in Brno (Czech Republic) - see link below.
You can use the probe of sheep in the electroejaculator in large dogs with a dose of sedative and pulses not more than 8 V, then, some dogs will response.
I agree with dr. Pavel Travnik, as there is no need for using electric ejaculation in collecting semen from dogs. The best and easiest method for dog semen collection is the digital manipulation technique. A trial of using the electric ejaculation can be indicated like that suggested by Dr. Magdy waheed mostly with more valuable and aggressive dogs.
answer according to size of probe for electroejaculation - yes the size must be different e.g. Minitübe provide different types of probes according to animal specie, also the voltage for electroejaculation must be adjusted for dogs.
Our team has also good experience with manual collection of semen as you can see in our publications. But as Mr. Elazab wrote in case of very aggressive dogs the electroejaculation is choice.
In male dogs, manual stimulation or the Artificial Vagina (AV) would yield positive response almost every time provided that the individual concerned is a healthy and non-agressive one.
However, when you change the collection environment or the person collecting the semen it would be problem. In these cases, using an Electro-Ejaculator (EE) would be inevitable provided that its voltage applied and the thickness of the probe used should be fitted for the corresponding sizes of dogs. In large breeds, ram/buck EE in size would be appropriate, while in small breeds tom cat probes would be more convenient.