Hi,
I'm guessing your image size number is the size of the PNGs you're importing. What happens behind the scenes is each of theses images gets converted into a texture suitable for uploading into VRAM, and if the texture is uncompressed, EACH 601x442 image would take up 1.013MB. So, 707 images at that size works out at 716.4MB. This is inevitable, and would happen in any engine you end up using - even if the engine loads directly from PNG, thats how much memory it would take on VRAM if the image is uncompressed.
If you try to import all these images and create atlases out of them in one go (assuming you've got multiple atlas spanning ticked), you will most certainly crash Unity.
The only way to work with data of this scale, is to split up the data into smaller and more manageable chunks, and create sprite collections out of these. My advice here is to avoid using the spanning feature, and manually create sprite collections out of as many sprites would fit in each collection. This number will probably be larger than the numbers just divided, as I'm guessing (and hoping) that your images would have a ton of empty space in them - 2D Toolkit will trim these textures by default, so hopefully you end up using less than the 700 MB worked out above.
Hope that helps.