IMO there's not much of a point in considering any one breed when it comes to one's autism. There are so many other pressing considerations in owning a dog.
Starting with whether or not in the present or near future you are a renter or an owner. Renters must be completely cognizant of their landlord's policy on pets and what the financial cost and terms of a pet deposit may be. Unfortunately for better or worse, there are also statistics out there relative to dog bite claims that drive both insurers and landlords to expressly prohibit certain breeds. Be very aware of this dynamic when considering certain breeds.
Then you must consider other things, like how the do might relate to being in smaller, confined spaces or large, open spaces. Especially important considerations in terms of breeds. A herding breed dog will thrive in large open spaces. A toy breed dog will likely do much better in an apartment. Though most any dog needs to be walked regularly.
And how often would you be with a pet. Being aware of the possibility of separation anxiety. Some pets do poorly in this regard, and thrive on constant companionship. Where when you look down, your little friend is right there nearly all the time.
In the most general sense I believe the consideration of owning a smaller dog will inevitably involve fewer unknowns than owning a much larger pet. It all comes down to what kind of living considerations you can determine for your pet to thrive in, rather than suffer from.
And lastly, to be aware of what breeds have what good and bad points relative to temperament around other dogs as well as humans, and especially physiology. And veterinary cost factors. Don't assume that large dogs have longer life cycles. Investigate, read and research accordingly. You may be surprised at what dogs live longer than others.
One thing for sure, no matter what the size of a dog, most if not all will have big hearts full of unconditional love. The kind you may or may not ever sense with other human beings. In this respect any dog is a good choice for an autistic person. Be it a big, little, purebred or a rescue dog.