I tried a 120gb SSD but for my usage - Visual Studio 2015 and a bunch of emulators - it was really too small. Luckily I found a Sandisk 240gb in Amazon's Black Friday sale for £40. On a laptop, you can buy caddies that fit in the optical drive bay to take a hard drive, so you can use the smaller SSD bigger HDD method. I've lost the DVD drive, but I hardly ever used it and I've not missed it.
We don't use our DVD drives on the laptops either, but fortunately, the one I upgraded is a 17.3" laptop, so has two drive bays. 1TB SSD and a 1TB HD now. Bags of space.
I got the big Sandisk SSD for £140 in an amazon sale (RRP £220), less £90 of vouchers from work when I had my bike accident, so cost me £50.
I picked up a 240GB for about £60 out of sale (Samsung Evo) for the old computer and my son's gaming machine has a 240GB SSD for the O/S and a 2TB HD drive for games, and a portable 2TB for backups/old files.