How do I spawn a large platform of lava up at build limit?

When liquids are manually spawned into the world, they can be still or flowing. A flowing liquid block will attempt to flow into adjacent blocks as per the usual rules. A still liquid block will not do this. Note that the flowing-vs-still distinction is completely separate from whether or not it's a source block, and has nothing to do with depth, the flowing animation, etc.

Still liquids are called "water" and "lava", while flowing liquids are called "flowing_water" and "flowing_lava". You want to spawn "flowing_lava" instead of "lava".

The reason Minecraft has two different kinds of block is because most of the time, liquids don't need to flow right after they spawn in the world (e.g. as part of an ocean or lake). It would be inefficient to trigger large numbers of block updates whenever new ocean terrain were generated. So Minecraft spawns still water which does not attempt to flow anywhere. But sometimes, liquids spawn in places where they do need to calculate real flow (e.g. waterfalls), and these are spawned as flowing liquid blocks. All flowing blocks transform into still blocks once they've done the flow update, and still blocks transform into flowing blocks when a block update happens in an adjacent block.

Try spawning a platform of stone below it then removing it by setting air where the stone is. This will trigger (two) block updates and will cause the lava to begin flowing. Good luck man!

To make a platform in the air, you should probably use the /fill command, put the coordinates in, and specify the block. However to make a platform that is not a rectangular plane/prism, you could either use /fill to get the parts of the shape that can fit prisms and use /setblock to get the rest or you could use /setblock for the whole design.

The main problem with the first method is that some shapes don't have a space that can fit prisms of one block type (like a diagonal line). Another problem is that if you want a design with two or more blocks, the /fill command can only place one block type in the coordinates given to it. If you don't have these problems then this method is best because it saves time and frustration from having to place individual /setblock command blocks.

The other method of using /setblock all the way solves the problems in the first method, but it uses as many command blocks as the object you are trying to create. This method is also a pain to wire up if you want to activate the command blocks at different times. To get around this problem, you could place the command blocks that you want to activate at the same time all together and then use the /fill command to cover all the target command blocks in redstone blocks to activate them at the same time without having to count the ticks on repeaters to activate blocks when you want to. You will still have to wire the "redstone block" command blocks your self, but it will save the hassle of learning exactly how repeaters work and how to use them. Keep in mind that blocks like water need to be updated when set (pistons that are set by commands do not go into a powered state if next to a powered block or object making useful BUD switches).

After triggering the lava/water, immediately have command blocks set the triggering blocks to air to avoid having the lava/water spread out into the plus like form. Remember to build the command block area away from the creation area to avoid destroying the wiring by any water, fire or explosions. I you want to remove the creation after using it or you want to reset it, just do /fill x y z x2 y2 z2 minecraft:air. The X Y Z and X2 Y2 Z2 coordinates are the edges of your creation that you want to reset/destroy. Another thing to remember is to test each step of your creation individually, that way if a problem arises, you know which segment it's in. If you want to, color code the sections of the command block area, it helps you organize the many redstone circuits you might use.