Size/breed chosen really depends on what the service dog needs to do for the handler. Most tasks that a service dog would be trained to do for a person... may be tough (if not impossible) for a smaller dog to do, which is why you normally see medium to larger sized dogs for most service work.
For example: A smaller dog will not be able to turn on/off light switches. A smaller dog may not be able to open or close doors. A smaller dog would not be able to assist a person in any physical way such as helping with balance, helping to stand after a fall, or pulling a wheelchair. A smaller dog may struggle to retrieve fallen objects, either because the object is too heavy/big or because they can't reach the person's hands to return the object.
COULD a mini or toy poodle be trained as a service dog? Sure. I don't see why not. They have the intelligence for it. The real question is - WOULD a mini or toy be ABLE to be trained to do what YOU need it to do? And that depends entirely on what you need the dog to be able to do.
I've seen some smaller hearing alert service dogs, for example. I've seen some smaller psychological service dogs also (though they can't do deep pressure therapy or physically block behaviors they can certainly be trained to provide emotional support and/or possibly be trained to interrupt some behaviors.)
What size is the dog you have now? Are the tasks that this dog does for you, something a smaller dog would be able to do as well? Start there. If you have a medical professional who prescribed this dog for you available, maybe they can also chip on on wether or not a smaller dog would be suitable. Or a trainer, or program advisor, if that's how you obtained the current dog. Speaking to a breeder might be worth a shot as well - see if their breeding dogs would even be suitable for the work (suitability is hit or miss depending on the individual dog as well - you'll want to talk to breeders emphasizing the importance that this dog will be used as a service dog so they can help you pick a puppy that has the best chance possible of being suitable.)
Good luck!