How big is a block in Minecraft?

It has been well established that the length of the sides of one solid block in Minecraft is 1m, making its volume 1m³. This is supported by Gamepedia.

The height of the player is 1.8 blocks, thus 1.80 meters.

The wiki also mentions voxels, which is kind of like a 3D pixel. Because the blocks use 16x16 textures, you could consider a voxel to be 6.25 cm on a side and to have a volume of 244 cm³.

According to the Minecraft wiki, Minecraft uses the metric system. Each block is 1 cubic meter.

A block is 1 meter (approx. 3 feet), half the size of a full grown adult however if you plan on making replicas or recreations I would adjust the height for your 7 year old. (Assuming they’re shorter than about 5’5)