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Now I can understand more of what you want to ask, it's about design method in Inventor for quick changing.
You can use Edit feature to change the length like the picture.
Or if you want to become a more advanced user. I would recommend you focus on parameter design. Go to Manage/Parameters and update dimension d10 from 1000 to any number you like. You should name this parameter when you input it in the distance box e.g length=1000. So you can find it easier in the Parameters table.
Parameter design means you can control the part that you have designed using a defined parameter table. So you can update the dimension quick and easy.