Yes that's a know problem with the current viewport. However notice that when some islands are added distributed on the tiles, if you "center all" (key A), you can see all tiles, the problem comes when the tiles are empty. But yes that should be improved, thanks.
"Also i tried to resize the viewport but it is locked to a certain size?". There is a minimum size that's true, but no maximal size. Do you want to be able to reduce the size ?
with the viewport i meant, can i move the division between 3d view and uv view left and right. image attached here ibb.co/mabK7F. i couldnt see a way of doing that but might be useful if i want larger space for just uv but want small view of 3d viewport still. actually i see now that i can make either view fullscreen and toggle between the two which could be another way to work with it.
btw. the shading group distribute across udim tiles is a great tool!