One of the only economic statistics available which are capable of meaningful analysis, given the complications created by the Syrian Civil War, is the exchange rate with the Syrian pound which went from 50 per U.S. dollar pre-Civil War (2010) to about 500 per U.S. dollar at present, an inferred inflation rate of about 1000% over 7 years (an annual rate of about 39% per annum).
It is hard to know how much of this shift is from economic contraction and how much is due to increases in the money supply targeted to revitalizing the economy. As a rule of thumb, the rate of inflation is approximately equal to the first derivative with respect to time, in units of years, of Money Supply/GDP, using a consistent and reasonably broad definition of the money supply (e.g. not just paper money and coins).
If the seven years of hyperinflation has been largely due to a contraction in GDP, this would imply a 90% contraction. In reality, some of the inflation is probably due instead to growth in the money supply (a standard economic tactic utilized by governments in collapsing economies), so the contraction is probably less than 90%, but exactly how much is hard to tell.
Per the statistics cited by @yannis all of the comparison countries except Venezuela have less inflation in the 2010 to present time period (slight deflation in Greece, 286% inflation in Libya, 235% in Yemen, but 42,904% in Venezuela).
In Venezuela, while there is a collapse in the real economy itself, the massive hyperinflation it is experiencing is almost certainly predominantly from growth in the money supply rather than predominantly from a shrinking real economy.
It is possible, although it seems implausible, that Venezuela's economy has collapsed more than Syria's, since some of Syria has been relatively free of regime change or combat during the Civil War and has had a relatively normally operating economy that hasn't otherwise collapsed, while Venezuela's economic collapse has been national in scope.
Certainly, however, Greece's economy (which has seen GDP decline 25% since pre-financial crisis) has declined less than Syria's, which is not included in the comparison simply for lack of reliable data.
But, it is still hard to even figure out what a proper measure of economic collapse in Syria means. Does it include only comparisons of Syria's economic output in places that were under the Assad's regime the whole time? Does it include comparisons of economic output pre-civil war to economic output in areas that Assad's regime controls now? Does it include all of Syria's pre-civil war territory compared to the economic output of that territory today? Do you compare the GDP of pre-civil war Syria to the GDP of the territory the Assad regime controls today?
How do you measure barter components of the economy which is what a large swath of that territory was operating on during much of the (still in progress) Syrian Civil War?
Does the Syrian economic get credit for foreign money funneled to the ISIL regime while it was operating? If so, how do you allocate that between formerly Iraqi ISIL and formerly Syrian ISIL? Does it get credit for Russian military aid? Does it get credit for the U.S. led coalition's spending and employment of troops and others in the North and East?