A 99

The base Spot DC to see someone Large who is not hiding is 0; see Table 4–3: Difficulty Class Examples in Player's Handbook on page 64 and also here. The size modifier opposing Spot when creatures are hiding is +/- 4 per size step (see here). It's not explicitly stated that these would apply as penalties to Spot rather than bonuses to Hide when the creature is in plain sight, but I find no other guidance in the rules. After adjusting the Spot DC for size, apply the usual -1 penalty for each 10' of distance. Putting these together as an example, you would see a Medium creature at 30' if you had a Spot score of 7 or better.

A 101 No.
Demiplanes

This catch-all category covers all extradimensional spaces that function like planes but have measurable size and limited access.
Only extradimensional spaces which are stated to "function like planes" can be transited via magic like Plane Shift. Thus you could access the result of a Genesis spell with Plane Shift or the like, but there is no stipulation in the description for a Portable Hole or a Rod of Security that these spaces function like planes. Mordenkainen's Magnificent Mansion explicitly disclaims access via such planar transit spells:
Since the place can be entered only through its special portal, outside conditions do not affect the mansion, nor do conditions inside it pass to the plane beyond.