Try the Commands Troubleshooting and Help page if you get stuff with server errors.
Minecraft Magma Cube NBT Data Tags
Using the summon command to spawn in a minecraft magma cube is simple, take the command and enter into chat and presto you have an magma cube.
How summon different sized Magma Cubes
Magma Cubes can be summoned at a range of sizes by setting the Size:1 tag.
Sizes 1, 2, or 3 are the regular spawn sizes. This example summons a large magma cube.
/summon minecraft:magma_cube ~ ~ ~ {Size:3}How summon a super giant sized Magma Cube
Magma Cubes can be summoned by setting the by setting the Size tag larger than the regular 3. A magma cube the size of 20 is a giant to see. These mega magma cubes will also split into smaller cubes more times, as each size will split into the smaller size all the way down. The limit appears to be Size:64
/summon minecraft:magma_cube ~ ~ ~ {Size:20}
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If you're looking for a particular summon mob generator you can link to these specific versions of the command generator.