What animal suits the first animal head transplantation better?

There are several possible options:
(1) mouse/rat.
Pros: (1) It's cheap, which is good for (1.1) large number of experiments, for (1.2) big statistics in one experiment, for (1.3) easier reproducing of experiments by other scientists. (2) It's better for ethical committees because mice/rats experiments are generally perceived better than experiments on bigger animals. It's also good for reproducibility purposes (more scientists willing to reproduce on mice than on say goats).
Cons: (1) Mice/rats are small. Will it make surgery much more hard or expensive? (2) Will it also make post-operation analyses/biopsies/etc harder? (3) Mice have evolutionarily diverged from human ~96mln years ago<ref>https://www.pnas.org/doi/10.1073/pnas.051611498</ref> or 85-95Mya<ref>https://en.wikipedia.org/wiki/Euarchontoglires#Evolutionary_affinities_within_mammals</ref> while divergence with old-world monkeys happened ~31mln years ago[1].
(2) monkey.
(3) dog. Cons: many people like dogs and would be angry even more than in monkey experiments case.
(4) sheep/goat
(5) pig is perhaps better than dog or sheep/goat. All these options are similar but experiments on pigs are generally perceived better from ethics viewpoint. At the same time many people think that pigs are more similar to people than dogs or sheeps/goats. Dogs, sheeps, goats and pigs have the same common ancestor with humans (the common ancestor of Boreoeutheria) who lived 90-107Mya[2].

Mice/rats are small. Will it make surgery much more hard or expensive?

"The rat OLT [orthotopic liver transplantation] is among the most difficult animal models in experimental surgery and demands advanced microsurgical skills that take a long time to learn" https://pubmed.ncbi.nlm.nih.gov/23524839/

Mice/rats are small. Will it also make post-operation analyses/biopsies/etc harder?

Typical 100g rat has ~7ml of blood. Typical 400g rat has ~25ml of blood.<ref>https://pubmed.ncbi.nlm.nih.gov/3965655/ Fig.1 at p.3</ref>