Easily! I believe the bomb you're referring to was codenamed "Tsar Bomba" in the west (RD-220 and Big Ivan were the russians names for it). This was infact a 100Megaton design scaled down to 50megatons by replacing the uranium damper with lead. Making bigger bombs is entirely feasible with the sky being the limit utilizing the conventional "hydrogen" bomb implosion trigger method. The problem is engineering pushers/dampers that can cope under the strain so that the bomb stays together long enough that you get the yield you desire (this is assuming you've ironed out the other kinks to make smaller yield "hydrogen" bombs work) and also keeping the design mobile. The Tsar required a modified russian transport plane to use and was a free-fall weapon. So anything above about 100megatons is going to exceedingly hard to use in a military/warzone setting effectively. The main reason no one has made bombs with yields this big/bigger is because of the large fallout spread they cause (they reach high into the upper atmosphere spreading the fallout globally). They also generate large amounts of fallout, this was why "Tsar" was scaled down as the design changes significantly reduced it's fallout yield (that of ~1.5Megatons instead of ~50Megatons of fission), without this is would have contributed 25% again of the current fallout generated by nuclear weapons detonations. The by product is it was one of the "cleanest" weapons ever made (by fallout per unit of yield). The design for full yield was still validated by the test, which was the whole aim of the exercise. As to if america and israel themselves can make one, I wouldn't doubt they could but the resources required in making such a weapon and the negative side effects would outweigh any benefits that such a detonation could generate. Conventional ICBM mountable nuclear weapons are more than capable of filling any military needs for the forseeable futures.

Hydrogen Bombs are enormous. A few years before the Reds did it, the US began testing H-Bombs. The first was a 45 megaton explosion, the second was 55 megatons. They tend to vary in power based on the quality of the components, and because the Soviets were not very wealthy(understatement), they were limited by that. The US was able to make much bigger bombs than the Sovs, although there is a point where it just gets ridiculous. As to your second question, I don't think there is any solid limit to the power of a thermonuclear device.