Seamless glissandi are one of the things that due to the unique construction of the Trombones can be executed very easily on them. However, their maximum span can only be a tritone (the range the slide covers between all in and all out (or rather first and seventh position)).
Additionally, the range of the glissandi might be limited as you need to be aware that the harmonic scale of the Trombone builds on top of the Bb two octaves and a whole tone below middle C and only notes included in the natural harmonic scale based on that Bb can be produced in the first position (with the slide all in). If you want to learn more about this, here are some excellent visualisations.
For instance, if you want to glissando downwards from the B a major ninth below middle C on a regular Tenor Trombone, this will not be possible, as this B can only be produced with the slide full out (see chart linked above). For the glissando to be seemless, you also need to stay on the same harmonic. It is possible to cheat glissandi while jumping to different harmonics but it will not give you a completely seamless impression.
Effective glissandi however don’t need to cover huge spans. Most glissandi on Trombones don’t expand over a third. In fact, even semitone glissandi can be highly effective. Generally, glissandi upwards are more effective than downwards as they also crescendo on their way up making the effect more prominent.