Say you had a large bomb - take the 50 megaton Tsar Bomba. You then proceed to place it in the center of a spherical tank containing liquid hydrogen, and then detonate it. Could you start a fusion reaction inside of that tank (and roughly how big would that tank need to be)?
When the nuclear bomb is ignited, the liquid hydrogen in the tank would evaporate instantly. The evaporation heat of liquid hydrogen of a tank of any reasonable size is negligible compared to the energy released even by a small (Hiroshima) 15kT TNT fission bomb.
Note after comment by @rob: That there won't be any significant fusion of ordinary hydrogen with hydrogen has been answered here: Fusion: Why deuterium and tritium?
The first ever thermonuclear device, Ivy Mike, contained a tank with liquid deuterium (heavier isotope of hydrogen, the main isotope, protium, has too small fusion cross-sections). The fusion reactions in this tank were ignited by a fission bomb.
Note, that the main problem for obtaining a significant yield from fusion reactions is to contain fusion fuel long enough for it to fuse, rather than being dispersed by the explosion prematurely (then this would be called a fizzle). So, while simply placing the bomb inside the tank with liquid deuterium will produce some fusion reactions, large yields require careful design such as Teller-Ulam configuration.