A little more on age.

Seniors are not off the hook either.  All age groups are sometimes guilty of choosing a dog for all the wrong reasons.  The most glaring is for protection of property.  This usually means buying some large breed like a German Shepard, a Rottweiler or some fang bearing mongrel that would put the fear into God let alone a would-be thief.
However, size is not the deterrent in most cases, the mere fact you are a dog owner is reason enough, and size has little to do with the reason you will be left alone.  Most crooks are after cash or small items they can quickly remove without being caught or discovered and any dog will thwart this kind of degenerate.
I have 5 Shih Tzu dogs and combined they total, one 80 pound Goliath.  I have no strange visitors entering my yard without a chorus of wild yapping's thwarting any attempt by crooks including, sales people, bible thumper's, politicians and even the police don't dare open the gate if the dogs are outside.
Size is not the deterrence as much as the noise the dogs make alerting you and your neighbors to a disturbance.
Ensure that you do your homework before you take the plunge into dog ownership so that you get both a loyal pet and one that meets your needs, financially.