For example the left tool bar options including the paint supports tool https://help.prusa3d.com/category/left-toolbar_210

From the look of your screen shot it looks like you have added a cube as a modifier.  A mesh modifier can be different types, I'm assuming you want it as a support enforcer (which would make the box blue toned, red for a support blocker etc).  You can bring up the tools to move the modifier mesh around using the standard shortcuts, like M for move, S for scale etc or you can use the equivalent tool from the left hand menu if you want.  
https://help.prusa3d.com/article/move-rotate-scale-tools_1914

While mesh modifiers have their place for support enforcing or blocking (they can cover/reach areas of bad geometry that are otherwise impossible to reach with the paint tool) they aren't the most efficient way for general workflow.  They also have to touch/interpenetrate the actual area/faces on the Object you want to block or enforce.

Again 

Thanks

see screenshot.

I selected add a box, it is opaque black in colour, I can move it around the build plate, but cannot alter it´s size as having NO X Y Z pointers and should  the box be green in colour. Color  ?  Duh me   How do I turn this feature ON. 

Any help appreciated.

I will give it a go.

My preferred method is to set supports for enforcers only and then ONLY paint the areas where you want support.  That way the amount of support is minimal and you can leave the setting active on your profiles as you define where support goes. 

The paint tools once you get used to them are very quick to use.  Using a combination of highlight angle and the smart select makes it very easy to define areas.

Also hardly ever use organic, just snug.  Organic support has several unaddressed bugs that can cause issues depending on what it generates.

Thinking supports could be treated as a separate set of objects or parts.

Painting out irrelevant selected computer generated supports then I have problems with. I have never been much good with a paint brush says the wife. 

My ideal would be to allow the generation of supports and to selectively delete what I feel are irrelevant branches and limbs in the part to be printed.

It seems like its all or nothing using supports, I have no control where or why they are generated.

For example I recently printed a 200 mm tall vertical wall square with 30 x 4 mm dia holes  at different z heights,

the preview looked like a plate of spaghetti, so I unchecked supports and the print was fine, just needed to cleanup the holes with a drill bit.

I saved 2 hours and a bunch of filament. 8 times out of 10, most of the branches fall off, even with Z lift set and added extra diameter settings at the branch ends,

default is 0.8 mm?  Naturally using no support gave me a print failure warning, but got away with a nice project in the final print.

i really could of done with a "selective" organic support on a bridge section , but I got away with no support lucky.