You'll need to know the final destination for the Como train so you'll be able to spot it at a glance on the departure boards when you get off the train in Milan. Be one of the first at the train's door with your luggage, ready to get off. But if the train's late leaving Venice, not to worry. The train to Como may be late, or there will be others you can take heading north. If you have reservations on the Como train, and you miss that train, you may be able to transfer the res to the next train.

There is no way to know in advance how far apart your trains will be as platforms can be decided last minute. This list may or may not be accurate.

[edit] Platforms
Trains to various destinations leave platforms as follows:

Platforms 1-3: Chiasso/Domodossola/TGV Milan-Paris/Milan-Turin (ES* AV)
Platforms 4-6: Turin/Cisalpino Milan–Como–Arth Goldau–Basel / Zurich
Platforms 7-13: Venice / Udine
Platforms 14-17: Bologna–Florence–Rome–Naples
Platforms 18-23: Genoa-Livorno / Ventimiglia / Parma /Cremona-Mantua /Milan-Treviglio-Bergamo
Platform 24: Operations

I doubt this layout will help any, but Trenitalia doesn't seem to have and pictures/maps of the inside track area since the remodel.
http://www.grandistazioni.it/cms-file/immagini/grandistazioni/volantinoBIGLIETTERIA_IT.pdf

Your best source of information is the conductor on your train from Venice. He should be able to tell you which platform your train to Como departs from, and how to get to it. The Milan station is poorly posted and especially does not favor strangers in a hurry to make a connection. Make sure the car that you board is labeled Como or some city further up the line.

> I doubt this layout will help any, but Trenitalia doesn't seem to have and pictures/maps of the inside track area since the remodel.


http://www.amoitaly.com/img/deposit/deposit_map_milano.gif
This 2-D map may be easier to see. I have used this station more than a few times, last time being 1 month ago.

Your train from Venice arrive from the top of the map. You get off the train and come down to the area where "shops" and "exchanges " are. There you will see the signboard that shows all the next train departures. As advised above, it would be useful to ask the conductor on the train and also know the destination of your next train for Como.