The width of a video menu is determined by the number of videos it allows. The latter can be specified by the number of video file pins you add to the Pin Collector.

Other than that you cannot resize the elements individually.
Also you cannon create vertical panels and cannot display the pin name currently.

I think that there's a room for alot of improvements in

control panel designing

How to add slider type of control to modify values of module properties?

also how to change min and max value of this slider?

You cannot add a slider that is directly inside a box.  But you can create a box that display one or more slider in the property editor when it is selected.

You have to create a Pin Collector, and make some connections.  To display sliders instead of numeric fields you have to insert Dash Scalar modules:

On each Dash Scakar you can specify the Min-Max values:

If you add SLIDERS to the control board, you will see it like this:

(note that the slider itself always displays values between 0 - 1,  but the real output will be in the interval you specified.)

so there is no way to show real min and max values  in Dash Scalar  control panel?

Currently there is no, but indeed in lot of cases it is desirable, so we'll add this feature.

What about trigger buttons how to control it ? also how to add it to control dashboard?
i mean for example something like play,stop and restart in video player

You can simply wire them into a Pin Collector, and add a # character before the pin names.

# in pin collector names means that you want a button instead of a property.  It can be used with Trigger, Logical and Integer properties.

Also as you can see there are some predefined button names that turns into icons on the control board. You can see and select these from a list when you add a Pin Collection pin using the Icon Keyword button:

If the pin name is not identical to any of these then the button simply displays the pin name as a text.

Any updates about Customizing control panel elements ?

Sorry, currently we cannot provide more customizations.