It doesn't have to be that tall initially.
Technology is proven (see Netherlands):
That's what portion of Netherlands that would be under sea right now without dikes.
Sea level rise is fairly slow process, just do 1-feet wall then add another feet every decade or two. Absolute worse case scenario at the moment is ~1.95 meter sea rise by 2100, most likely theories go with 0.6-1.0 m rise, so, about six feet tall wall in 100 years.
For reference, Great Chinese Wall was built in 200 years with quite low technology level - and it was much taller.