This is a surprisingly common question, the answer to which is unfortunately really "it depends". Heck, Apple doesn't even bother telling you how many songs "should" fit on one of the new iPods in their advertising because it's so puzzling.
Nonetheless, there are some basic assumptions we can make about the size of different types of information. Digging through my own video collection, for example, it appears that an an hour of video encodes with reasonable quality to around 350MB, so if you want to get the latest episode of Family Guy and a classic re-run of Friends poured onto your Sony Playstation Portable (PSP), then you can assume that each 30 minute episode, if you don't edit out the commercials, will be about 175MB or so. Trim out the ads and you can cut that down another 30%, to around 117MB.
Looking at my own archives, my big fancy digital camera (A Nikon D80) takes pictures that are about 3.5MB each, but more modest digital cameras tend to take pictures in the 1MB to 1.5MB range. Further, while audio files are all over the place in terms of size due to encoding variations and differences in song length, it appears from crunching my library data that 5MB is a typical song length and since there are usually 10-12 songs on an album (oops, uh, CD), that we're talking about 50-60MB for a CD.
This means that...
A 512KB SDcard or flash memory stick can hold a bit less than three TV episodes (or one hour-long episode and some funny adverts, I suppose), or about 350 photos or 102 songs, which means about 10 CDs.
A 1GB SDcard or flash memory device can hold twice that, and, of course, a 2GB device can hold twice that again. In other words, if you were to pony up for the 2GB device, you could store eleven 30 minute TV shows (or a couple of feature length movies), over 1300 photos, or 400 songs (aka 35-40 CDs).
That's why I've personally migrated with my removable storage to using a Corsair 8GB USB flash device and am just about to upgrade to a brand new 32GB device. With the new unit, I can drop a few days worth of movies onto the device and/or tens of thousands of photos or hundreds of hours of music. A bit spendy at about $200 in the retail channels, but nice. Of course, this doesn't help you if you're looking at an SDcard, but companies like Sandisk do have 4GB SDcards, so that might be a good investment for your Sony PSP to enhance your viewing and listening pleasure.
However you do it, good luck and don't forget to check out all the other Sony PSP Help here on my site!
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The size of a multimedia file can easily be calculated by it's bitrate. Most multimedia files (audio, video) are encoded today using Variable Bit-Rate (VBR) instead of Constant Bit-Rate (CBR), hence the calculated file-size should be taken as an average figure.
For ex: if I choose to encode a audio-cd (MJ's Dangerous) at 128 Kbps (Kilo bits per second),
128 kbps = 128 * 1024 bits per second = 131072
131072 bits per second = 131072 / 8 = 16384 bytes per second
16384 bytes per second = 16384 / 1024 = 16 Kilo Bytes per second (16 KBps; note the capital B).
So for every minute of audio, 16*60 = 960 KB will be used. Since the full album is approx. 77 minutes in length, 960 * 77 = 73920 KB will be occupied or
73920 / 1024 = 72.1875 MB (Mega Bytes) disk space will be required.
Vary the bitrate and watch the filesize increase/decrease. The calculation hold good for video files too. For ex: MPEG-1 files (VCD Standard) are encoded at 1152 Kbps. For 1 hour of movie, 1152*3600 = 4147200 Kbits are required or 518400 KB (4147200 / 8) or 506.25 MB (518400 / 1024) are required.