You can use a cast net and i think the size is no bigger than 4 foot diameter. I would not suggest leaving them in a live well over night. Herring (especially the first run alewives) are not very hearty to begin with. I fill my bait tank with salt water( even though I am pulling them out of freshwater), catch my 35 and then tranfer them that day to my holding pen. When herring are put into captivity they "purge & piss" so you dont want them sitting in that water to long.
Using a cast net is a lot more fun than a dip net in my opinion and I have a great supply of 4' nets in stock.. For the ultimate HERRING net I have the 1/2" mesh heavy net that will throw easily and sink fast. I've caught a limit in one throw. I also have the best price you will find anywhere on the basic 3/8" nets.
I also have all the live wells, areators, bait cages and the like that you might need .
If you can't catch them or don't want to spend the time I also have live and fresh HERRING for sale. Hopefully they will be available thru May. Fresh HERRING will be done when I have to pull the nets at the end of April.
As far as keeping them overnight the more room you give them the better chance you have. If you have treated salt water and plenty of areation you might be able to keep two dozen in a 50 gallon live well water.