Just some quick thoughts on why I think conservatives should support small business:
1. Private property gives a person a measure of economic power (i.e., the power to access and use his own resources as he sees fit), and thus a measure of freedom. This power can be used to achieve one's ends in cooperation with others, or by coercing others.
2. A person who owns no property has no economic power, and thus no real freedom. (His freedom is ultimately contingent upon the cooperation of those who own property, and thus he is ultimately controlled by those individuals.)
3. The best way to prevent the abuse of economic power is to encourage a system which pits power against power, an "economic" checks and balances.
4. The socialist response is to promote the common ownership of property, but this merely creates a situation ripe for the tyranny of the majority: "Society" is somehow to control the use of property*; but what check is there to prevent "society" from abusing its economic power?
5. The conservative response is to encourage widespread property ownership, and thus allow each person's economic power to act as a check on that of others. In practical terms, this would entail conservative support for small business.
*How this would actually work is rather vague: Would it be a simple majoritarianism? Some sort of Rousseau-inspired "general will?" Bureaucratic control supposedly on behalf of "society?" Whichever is the case, there's no real check on whomever it is that possesses ultimate economic power.