At what points do vassals become too big?

I am playing as Venice, and I have Serbia and the Byzantines as my vassals. I am in the process of annexing Serbia, but it is a slow process. At the same time, I want to wage war on the Ottomans, and make them return provinces to Byzantines. However, I am a bit worried that they will become too big to be a vassal, and rebel or declare independence.

Here's how my realm currently looks – I am not sure how much is relevant.

COUNTRY    PROVINCES    FORCELIMIT    NAVAL LIMIT    INCOME    TECH    BASE TAX
Venice        17            44             60          44      9/6/7      114
Byzantium      6             9             20           8      7/7/7       24
Serbia         5             8              5           4      6/7/8       16

If we are looking at size (number of provinces), they are currently 0.35 times my size. I am planning to return 4 provinces to the Byzantines. That means they will be 0.59 times my size, which sounds worrying. If I wait untill I've successfully annexed Serbia, they will only be 0.45 my size (if no other provinces are gained or lost).

So, which numbers are relevant in the equation? Is there a set limit where they will start rebelling? I can see they already have a CB, but so does Serbia. Or will they simply declare independence and that's it?

EDIT: After doing some research, I've found out that it might have something to do with base tax, so I've added that to the table. I believe the magical limit may be 60%, but I am not sure. If that is the case, the math is as follows:

Current: 14%
Future: 43%
+ Serbia: 38%

Is this correct?