Try the Commands Troubleshooting and Help page if you get stuff with server errors.
Minecraft Slime NBT Data Tags
Using the summon command to spawn in a minecraft slime is simple, take the command and enter into chat and presto you have an slime.
How summon different sized Slimes
Slimes 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 slime.
/summon minecraft:slime ~ ~ ~ {Size:3}How summon a super giant sized Slime
Slimes can be summoned by setting the by setting the Size tag larger than the regular 3. A slime the size of 20 is a giant to see. These mega slime 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:slime ~ ~ ~ {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.