It's the biggest Republican majority since the 1920s. You only have to go back to 2008 to find a larger (Democratic) majority of 256 to 178 in the House and 60 Senators (for a short period).

The House has had a 435 members since 1913 (excepting short periods when temporary members were added at the admission of new states). This is set by statute. The largest majority in the House occurred in the 75th Congress (1937-1939) with 333 Democrats, 89 Republicans thanks to FDR's 2nd term coattails.

The Senate has had 100 members since 1961. Senate seats, as you probably know, are 2 per state per the Constitution. In the 89th Congress (1965-1967), Democrats held 68 seats and Republicans 32 thanks to LBJ's coattails. However, the most lopsided majority I know of is also the 75th Congress' 75 Democrats to 17 Republicans.