How to light up a huge room without mods

Solution 1:

Light comes from light sources (like Torches, Fire, Lava, Glowstone, Sea Lantern, campfire, beacon, Redstone lamp, lantern etc.) or from the sky.

Depending on what you want to achieve there are different ways to help you.

If you want your whole hall to appear uniformly bright you will most likely need to have a design featuring an open roof (since we are talking giants this would not be unfitting).
Otherwise there are plenty of videos out there that feature tons of design tips about lighting up you builds. Here are just some of the most common I can think of:

• Have thematic correct light sources (In you case chandeliers hanging from the ceiling or a big fireplace).
• Place carpet over light sources (this will not block the light and you can walk on the carpet.
• Place light sources under other non-full blocks that still hide at least part of your block (stairs or chests for example).

If you only care for spawn-proofing, meaning that you do not want any mobs to spawn in your hall, you have the option to cover any space that is not lit up with spawn-proof blocks (glass, carpet, half-slab, etc. [not sure if Soul Sand works in Bedrock Edition]).
For more detail you should look into the "Spawn" article on the wiki.