Just multiply to get the height of the sidewall. On your 195/70's, multiply 195mm x .70 to get 136.5 mm.(the height of each sidewall)

So if you were going to get 245's, you'd want 136.5 / 245, giving you .557..... so to get the same height tires, you'd buy 245/55.

235's would 136.5/235, or .58, so you could go either 235/60 or 235/55.

In either case, the sidewall would look the same as what you've got on there now, your speedo would be right but you'd have the bigger footprint that you want.

Somewhere on AF, there's a link to a page where you can do the calculations; I couldn't find it in a reasonable amount of time so put the calculations above.

Hope this helps out