Slippers of Spider Climb to climb huge creatures

[GNiessen10]

As long as the skin of a creature is not slimy or slippery, can a character climb up a creatures back that is 3-4 sizes bigger? For example could a gnome climb up the back of a T-Rex? The slippers do not require a climb check or skill roll. And you don't need to have your hands free. So it seems to me that one could stand on the back of a T-Rex and be safely out of biting range.

[Quandary]

This is wholely within houserule territory.
Likewise the concept there is anywhere within reach or same space that creature cannot attack is houserule,
as normal Line of Effect is calculated from any corner of creature space to target, given facing does not exist.
There may be other RPG systems that handle this is more cinematic manner.

[Anonymous Warrior]

Generally, spells say what they do, and don't do anything they aren't described as being capable of doing. Spider Climb is listed as allowing you to climb surfaces, but doesn't say anything about creatures. Moreover, the slippers don't really do anything but give you the benefits of Spider Climb. So that's a probable no on that count.

It also doesn't help that somewhere there's some class ability or feat that allows you to do what you're describing. I think it might have been one of the Swashbuckler archetypes.