By creating folders you can do a lot in the speed dial, but if you just want to toss a lot into it (without thinking too much about the folders to follow) there are several possible strategies. Let me say first off, I'm not sure what the limit is on the speed dial. I clicked in settings, and the max there is 9 columns, but I seem to only be able to get 8 when I try. Going down, rows, perhaps it keeps going. I was only able to get 6 down. Didn't need more, so didn't try. It may -- haha -- go forever. You might want to test it. In any event, no big deal, there are alternatives. You can also toss the mass of items onto your personal tool bar, or using a bookmark extension (make bookmarks of it, and from there take a bit of time deciding how to organize your speed dial. My approach -- an you may be different in what you want -- is to put (in addition to folders or items I already have on the speed dial (right now, about 18), 2, 3, 4, 5 quick items on the speed dial at the end of that, because I know I will make instant folder with it, or move those items into an already existing folder. But if it's more than that, I use the personal tool bar (bookmark items to it and then drag to the speed dial), particularly when you are ready to make a folder. Remember, putting a speed dial in each folder -- you can organize tons of sites, though the speed dial is limited to one page. (If you want to test how many rows you can make, let me know!!!). Maybe you can keep going down with rows
I would make the speed dial icons small. (Go to settings, Browser, and for the Start Page, do not put a check in box about big thumbnails) (the bigger the thumbnails, the fewer the columns you can get, I think. Now beyond that setting, there's a way you can get more columns. I'm not sure why I only get 8 columns instead of 9. I think to get the 9 columns, you need to adjust the speed dial thumb nail width and height by going into Power Mode (which we have explained in the past this forum how to do). You go into settings, click outside the search box, then use the arrow keys: up up down down left right left right b a. Here's a link on it: http://newsblog.wti.com/index.php/2013/11/opera-19-developer-update-how-to-enable-operas-power-user-mode/
But remember to click outside the search box first, before you start with the arrow keys. (A thought: if you adjust the settings to make the speed dial smaller, it's possible you get more columns. (You may want to remember what the settings are that you start with, just in case you want to return to them). I vaguely remember a post, where I thought someone said the max. no. of columns is 11, though I'm not sure there. You can also set up nice wallpaper for the speed dial, even custom wallpaper from your own computer, to make the speed dial look great, but that's another issue for discussion..
Remember, everything does not have to be in the speed dial. You have a personal bookmarks toolbar, can use your bookmarks extension (you should download one), plus you have Stash for some bookmarks -- things to save. I hope this might be of some help, and give you ideas.