The 7(a) Loan Program is the primary loan program used by the SBA to aid small businesses. The loans made under this program are generally flexible since financing can be guaranteed for a range of business purposes, such as: machinery and equipment, land and building, working capital, leasehold improvements and furniture and fixtures.
Usually 95% of all businesses are eligible for SBA financing as long as they are a profit seeking organizations. Typically a business is eligible if it is either a retail or service company with annual sales under $21M, or a wholesale company with less than 100 employees.
More specific size standards can be found here: http://www.sba.gov/regulations/121/
The SBA Eligibility Questionnaire for Standard 7(a) guaranty can be found here. (Warning: This form is not short and hence may be quite painful to complete) http://www.sba.gov/idc/groups/public/documents/sba_program_office/bank_eligibility_questionnaire.pdf
If small business owners are looking to refinance existing debt, reimburse funds owed to any owner, repay delinquent state/federal withholding taxes/funds, make a change that is non-beneficial to the company, or use the loan for what is deemed any non-sound business purposes, then they should not apply for a 7(a) loan.