It's possible to make custom blocks without resource packs in Minecraft using item displays

Custom-Blocks-Without-ResourcePacks.md

Custom Blocks without Resource Packs (Minecraft 1.21.4)

It's possible to make Player Heads the size of full blocks by putting them into an item display,
then changing the scale of the item display to 2 on every axis.

This is useful because Player Heads are the only block that you can change the texture of without a resource pack,
as the texture is stored in the item components as one (or both) of the following:

1. The username of the player whose skin should be used. (as seen in the example command)
2. A Base64-encoded link to a skin texture on Minecraft's servers.

For more info, see https://minecraft.wiki/w/Player_Head

Example command (for Minecraft 1.21.4):
/summon item_display -6 -60 1 {transformation:{left_rotation:[0f,0f,0f,1f],right_rotation:[0f,0f,0f,1f],translation:[0f,0f,0f],scale:[2f,2f,2f]},item:{id:"minecraft:player_head",count:1,components:{"minecraft:profile":{name:"MHF_Cake"}}}}