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@M. Frary said, usually to create a bonsai, first a seedling is grown to 5 to 20 times the height that the finished tree will be, then the tree is cut down to size. Most building their own bonsai simply shop for the tree that is already 10 times taller than the finished size, so that is what Mike means when he says we don't grow them up, we chop them down to size. Actually the height of the tree is not critical, it is the height that is needed to get the diameter trunk that is desired. Thin trunks give you a tree that looks like a seedling, a thick, 2 to 6 inch diameter trunk will give you a tree that looks like a tree one a one foot tall tree. To get these large caliper trunks, we often ''shop'' the trees that are 4 feet to 12 feet tall. Depending on species, sometime much taller, sometimes a short tree can have a large caliper trunk. It is not the height, it is the caliper. Nearly 100% of all the bonsai you see in photos spent at least a few years in their early life as a much larger tree. Then they get brought down to size.
Sycamore does not have a good track record as bonsai, it has been tried by many and I have never seen a photo of a convincing, reasonable quality bonsai that was a sycamore. Not saying it is impossible, but the species has a number of traits that make it difficult to ''bonsai''. Big leaves, coarse branching, if you were to be successful at all, shoot for a finished size of roughly 4 to 6 feet tall and wide. Smaller than that and the huge leaves will always look out of proportion.
If you want to practice, and sharpen your horticultural skills, then just get these seedlings growing and see how large you can get them to be while still keeping them in pots. Keep them going for 3 years and you will have proven you can handle basic horticulture.
In the mean time, get yourself a 1 or 3 gallon nursery pot with a juniper. Procumbens nana juniper, or shimpaku, or just about any species of juniper carried by your local landscape nursery. Also a flowering quince. They are nice and easy and hardy outdoors, year round, almost anywhere. Both juniper and flowering quince,(Chaenomeles is flowering quince genus name), once in a 3 gallon nursery pot, will increase trunk caliper, in a reasonable time, while still in a nursery pot. Flowering quince is often used with a relatively slender trunk, as it is appreciated for its flowers, and ''tree-ness'' is not as critical a display requirement for flowering quince. There are many good species to try, read through the various sub-forums on BNut and see what others are working with. Try what ever species that appeals to you and seems likely to survive in your climate for outdoor trees, or has a proven track record as a houseplant. Don't try trees that people repeatedly say ''won't work'' - it is not because we are snobs, it is because we are trying to save you time and effort, as we have already tried and failed. Benefit from the failures of others, try something that is proven to work.