Well, a DVD-R/DVD+R is meant to be 4,700,000,000 bytes, Normally it is a few bytes over, but not all the time so it is best to start working from 4,700,000,000 bytes.
A kilobyte is 1,024 bytes, and a megabyte is 1,024 kilobyte, and a gigabyte is 1,024 megabytes. So 4,700,000,000/1024/1024=4482.269287109375.
So lets say a DVD is 4482MB. Many programs (IE Nero) refuse to burn higher than 4480MB without overburning so lets say it is 4480MB. Therefore in theory you could go right up to 4480MB. However, DVD Shrink often does not hit its encoding target properly, and also remember that the last bit of a DVD is often a poorer burn, and the outer part of a disk (where the last bit of data would be stored) can easily become damaged. Therefore I would recomend allowing between 25 and 100MB spare space, so 4400MB seems like a good amount to go for. A few extra megabytes won't noticably improve the quality anyway.
So no, I would not try and go any higher than 4400MB.
As for your second question, if it is a DVD-5 then yes, it is better to use DVD Decrypter to Read/Write the disk, this will ensure a true 1:1 copy of the original. Even without using recompression DVDShrink can alter the DVD in ways that you may not allways want. Only use DVDShrink when you have a DVD9 you need to fit on a DVD-R5.