"standard size glass"??

is there any such thing as standard window sizes, or are they made to suit?

I ask because I have a cracked double glazed unit in a side opening window and want to order a new unit..
from watching them fit the windows it doesn't seem that difficult, just prise the plastic beads out of their grooves, pull out the old unit and refit the new one in reverse order..

or do I have to silicone the unit in?

ours had doube sided sticky tape to hold the window unit in place, then as you say, beading.

but i believe they are all custom made to order