I love pines, especially pinus sylvestris, pinus parviflora and pinus cembra.
But for some reason my needle size is huge, even in my good soil mix.
I decandled my pinus sylvestris like showed at mirai, but the parviflora I left untouched.
What else can I do, to smallen the needle size?
How long have the pines been in their pots? It takes a few years for the confined environment of the pot to really help reduce the needle length.
I agree with @nmhansen, it takes a few years for the needle size to reduce. Keep doing what you are doing but really what you need most is patience. 
OK, so I think it might be a good idea to have a look at the video content for single flush pines. You shouldn’t be decandling a Sylvestris if you want it to survive long term. They are a single flush species, decandling is reserved for multi flush pines like JBP and JRP.
In terms of needle size, it should only really be of concern if you are looking to transition your trees into refinement. In development, allow the spring growth to elongate and harden off. Doing this should see budding result along the branching, so you have options to prune back to after this growth has hardened off.
When you are looking to refine a Scots pine there is a pinching technique you apply throughout the growing season that helps to distribute energy across the buds evenly and increase ramification/growth evenly across the tree. This will help with needle length etc…
Pinus Parviflora are a different again in their treatment. I don’t grow these so will leave that to others with more experience to comment there, but again I know if you do a search in the mirai videos for Japanese White Pine you will find a lot of very informative and useful info in the video content.
Hope that helps a bit.
That makes sense. They are in their pots for 1-2 years. I’ll wait a bit more then 
Yes I think I meant pinching then. I followed the guide in the video. I leave about a thumbnail size of the candle und twist the top bit to relocate the growth to the inside. The new needles are super long though.
Not sure if it is climate/weather condition related, but minor also have rather long needles this year.