There is no standard size for a control panel and that is also true on the real railways even though there is standardization of components.
A control panel is as big or as small as it needs to be to control the area it controls.
In model terms it can be as small as 75mm square and only control one set of points or a monster panel controlling a major terminus and carriage sheds.
Also it depends on how you want to control the layout do you want just one panel or more than one panel.
Many people mark out the panel with auto pin stripe for the track others do a paper print out and put perspex or other clear cover on it some just draw one.
Some use an aluminum plate with pinstripe (me) not done one for my current layout it just a bit of aluminum angle with switches and levers on it.
It was only supposed to be a temporary prevent insanity layout but has lasted longer than expected so it may yet get a proper aluminum panel.
The best way to work out how big you need it to be is to draw it.
It doesn't have to be anything special for this sketch but you can then lay out the switches showing how big the base is and is there enough room if not time for a bigger sketch until you hit a size that works for you.
Don't forget the extra bit around the edges for mounting the panel that can't be intruded on with switches or wiring.