I understand that during salaat, for men - the entire region from above the navel to below the knees, should not only be covered, but its shape should

Question

I understand that during salaat, for men - the entire region from above the navel to below the knees, should not only be covered, but its shape should also be concealed, i.e., the clothing should not be skin-tight. Without this, the salaat is not valid. However, there are two issues I am worried about: (a) The lower garment - I understand that the trousers should be loose and not tight-fitting or hugging closely the thighs and knees. But how loose is good enough? Is it considered sufficiently loose if there is enough free space in each leg of the trousers to fit an arm alongside the leg? This understanding of looseness becomes important when taken together with the below problem about the upper garment. I wish to understand if the validity of the salaat is affected by the situation. (b) The upper garment the kurta which covers till below the knee during standing position in salaat, rises up when in the ruku' position. Although it still covers completely the posterior and most of the thighs, the problem is that the kurta rises up to about a few inches above the knee. During sajda, there is no problem as the back panel (daaman) of the kurta reaches the upper end of the ankles, and the thighs are folded inwards. However, during jalsah, again the kurta slightly rises - partially exposing the knees.