Friedrich Schneider is a leading researcher for compiling cross-country data on the size of the unofficial (or shadow or informal) economy. His latest paper (Schneider and Buehn 2009) presents the data for 120 countries over years 1999 to 2006, which seems to be an updated version of Schneider (2007), which in turn seems to be an updated version of Schneider (2005).
See also Friedman et al. (2000), who use different measures of the unofficial economy than Schneider and find that the size of unofficial economy is correlated negatively with tax rates and positively with corruption.
See Section 6 of Schneider and Enste (2000) for various methods to estimate the size of the shadow economy.
For cross-country data on the size of employment in the unofficial economy, see the reference in Table II of Djankov et al. (2002), who find a positive correlation of this variable with the degree of entry regulations.