Map size walk through walls is a possible walk through walls method in Pokémon Red, Blue, and Yellow, requiring expanded item pack.
1. Go to this spot in Vermilion City (other locations within the city are fine, however this spot will avoid the side effects of lag items due to the 0x50 sub-tile early on the screen)
2. Find the Hyper Potion x20 in item slot 38, use Select to bring it up to items slot 1-20
3. Swap a Hyper Potion x81 in item slot 38 (avoid swapping two stacks of the same item with themselves, or the game will attempt to merge them)
4. Save and reset the game. The player will now be in a Glitch City.
5. Dismount if Surfing. Walk through walls is now active.
6. Move north until in Route 6.
7. Encounter a wild Pokémon and run (fixes broken Start menu position)
8. Move where you need to with walk through walls (the next steps will disable it again). Entering a building will break walk through walls, so keep this in mind.
9. Open the menu with Start and then save, reset to bring things back to normal. Do not close the menu without resetting (reset as soon as the message says the game was saved or any time after text starts to appear as Yes/No box disappears) as a map specific side effect may occur that could freeze the game (on Route 6, it may reload where you were in Vermilion City).
10. The player can repeat this glitch after duplicating the old Hyper Potion x20 with old man trick and adjusting the quantity to x81 again