Don't put the cart before the horse. Write the episode first. Then you can work through how to make it meet your needs. Trying to anticipate everything before writing tends to inhibit being creative. You can
always break it up as needed later. If the main characters are interesting, the audience will usually give you the benefit of watching the second episode. My suggestion is to write a good episode. Then, if you have to, split it into two parts at a nice tantalizing juncture.
As a guide (not a rule!), action happens every five pages with major action on the 10s. Please don't take that as an exact page count. Typically one page is roughly one minute. For television, that generally means on the television breaks. Every ten minutes is roughly each "act transition". Since most webisodes are 10-15 minutes long, you will have 2 to 3 action sequences. Usually the final sequence is the cliffhanger.
Write your 10-20 page script. Then go back and check that every 5 pages or so, there is a major action. Then if you're longer than 18 pages, look for a good place to split around page 10 or so with a cliffhanger.
Most characters should have been introduced by then anyway. You have time to develop them more fully, just highlight and accentuate their foibles (for a comedy) that makes them likeable and worth watching again.
This will sound rather simplistic, but watch some cartoons. Writing for cartoons is HARD because of the pacing. Old Saturday morning cartoons, modern adult cartoons. Watch how they pace action. If you had never seen Bugs Bunny before, how does the writer pull you in? Most older cartoons are only 5-8 minutes long. Even Family Guy or Simpsons tell a lot of story in a half hour.
Remember that your characters become known to the audience not only by what they say, but what we see around them, how they dress and how they act. One page of script is just the outline. As director, you can amplify and convey a lot of information visually.
Code:
EXT. WEBSTER BUILDING PARKING LOT - DAY
A mazzerati pulls up next to a porshe. The door open and a long shapely leg steps out.
A man in a tuxedo and sunglasses sits in the porsche, glances over feigning disinterest
as his eyes trace upward from her ankle.
MARGARET
Oh, Robert, you're such a bore!
She takes her purse and slams the door.
She starts towards the condo elevator, her hips shifting with each step.
He starts and revs the car, pulling back. He glances and sees her in the
rearview mirror tipping down his sunglasses.
ROBERT
(under his breath)
Right, baby. You want me.
The porsche races off.
A smirk creeps across her lips.
The bell rings as the elevator opens. She nods to the operator.
She starts to enter and totters on her high heel before toppling to the side.
MARGARET
Damn!
The door closes.
Less than half a page but there is a sense of these two characters, their lifestyles, something of their relationship. The reader/viewer has enough to make guesses but not enough to know anything yet. But importantly, I don't have to explain that Margaret and Robert are both rich. That they know each other socially. In the next five pages, I can build the links by showing them in action in their surroundings. Then boom, I introduce the conflict around page 6 that set up the rest of the story. In shorts, it is especially important to show more than you tell.
But write the episode ignoring everything else. That needs to be your first step. Then go back and do the fine tuning. Good luck!