DavidZ - It depends what you are looking for. These are giant sequoia, which are found in scattered groves on the west slope of the Sierras, from about 3600-8900' elevation. I have been in many of the groves, from Calaveras Big Trees (one of the northernmost groves) through Yosemite and Sequoia/Kings Canyon. By far the best examples of giant sequoias are found in Sequoia/Kings Canyon National Park. If you don't have time for extensive hiking, Giant Forest and Redwood Mountain grove are really good options.
If you want taller trees, you need to go to the coast to see the coast redwoods (a different species). Giant sequoias will grow up to 40' diameter and 321' height (typically 25-30' diameter and 275' height for a mature tree); coast redwoods will get up to 379' height, but are typically 20' diameter for a mature tree. There are a number of good places to see large coast redwoods - Redwood National Park in northern California, Prairie Creek Redwoods state park (where Return of the Jedi forest scenes were filmed), and Big Basin Redwoods state park near Santa Cruz.