According to the SBA.GOV website, as of 2018:
The SBIR/STTR Statute (15 U.S.C. §638) establishes a cap for the maximum dollar amount of SBIR and STTR awards, above which an agency must request a SBA-approved waiver. SBA shall adjust the maximum dollar amount every year for inflation. The adjusted cap is effective for all solicitations and corresponding topics issued on or after the date of the adjustment. Agencies may amend their solicitation and other program literature accordingly. Agencies have the discretion to issue awards for less than maximum dollar amount. For more information regarding a specific agency’s award guidelines, please visit their solicitation and website. Agencies may exceed this cap for a specific topic with approval from SBA prior to the release of the solicitation, award, or modification to the award for a topic issued on or after the date of adjustment.
As of 11/20/2018, agencies may issue a Phase I award (including modifications) up to $252,131 and a Phase II award (including modifications) up to $1,680,879 without seeking SBA approval. Any award above those levels will require a waiver. Agencies considering this authority should review §7(i)(5) for additional information.