As flyingfkb says, the train ride from Venice to Munich will take all day. However, that is a scenic train ride, through part of the Italian Alps, the Brenner Pass and then the Tyrol in Austria.

Personally, I'd split the trip with three days in Munich, three in Venice and one for the train ride. However, this does depend on what you are interested in doing. There's more than enough to keep me happy for a week in either Venice or Munich, especially if one includes day trips by train. If I were you, I'd just think about what sites in or near Venice and Munich I most want to see and select the number of days in each accordingly. However, you can't really go "wrong" here as there's lots to see in both cities.

One thing I'd consider as a possibility is that one can do day trips by train from both cities to world-famous sites. Padua is an easy trip from Venice. Neuschwanstein is a longer day trip from Munich but hundreds of people do it every day in the summer. Dachau isn't "enjoyable" but it's something you're unlikely to forget.

If you dislike walking to the different sites, Venice can be a tiring city. When I was there, I walked everywhere and it seemed to me that many locals who live or work in the old part of Venice did the same. Venice can also be jammed full of tourists in the summer in a way that Munich isn't.

Given the long train ride, for Munich, I'd get a hotel near Hauptbahnhof but if you do that, you should know that part of the area to the south of the station are a red light district. If I stay there, I use google streetview to make sure my hotel isn't next to a strip club or something. Having said that, I've stayed in that area several times and never had a problem.

As someone who doesn't live in Europe, my opinions can be out of date or even just flat out wrong. I always welcome correction from people with more up to date or accurate knowledge.