Had this same issue:
Selecting them all is possible by selecting the fields LIST on the right menu, under TOOLS >> EDIT >> FIELDS. You sohuld see a list of all the fields in the document and you can select them all.
First double click on a random field, to bring up the properties window. Then Select all the fields in the list that you want to modify. Make the desired changes, and make sure all the fields in the list are highlighted.
Selecting them as a group in the actual document didn't work, for me at least.
If using the auto-detect fields option remember Acrobat tries to guess which are multiline and which aren't, so it's good to double check. You can use the same tip as above if you want them all the same: select the desired items from the list, then click the multiline option once to set them all the same. Now you can set them however you want.