I personally started with the prebuilt Ritter PSK and then worked my way to customizing. I think While having a starting point at size helped I originally downsized to a container I saw and then upsized to its identical big brother.

Personally Id start with a altoids tin size right off the bat. Its not to big not to small and its very common to obtain and very commonly used for these reasons.

After you get use to it Decide for yourself wether you need to upsize, Downsize, Change shape, Or Even change materials.

Bringing up a great point is literally today I saw a small pill case for 1$ at walgreens and bought it without a huge idea on what to put on it. My final choice was a fishing kit as i do not have a tiny Fishing kit yet.

One Idea is to purchase the major Important items OR Items that You may consider being the largest items first. This way you have a starting point for size to consider. Three great items for this idea is the Fox 40 Whistle the Ritter Mk5 and the spark lite.

This will set the tone for your PSK's size while adding some good solid basics to it right off the bat.

As a Quick note, I am a Container Junkie silly as it may sound. I often Buy containers just for the sake of having them around and most of the time I will think of a Need and instantly go through what containers I own and 90% of the time I will have the perfect container that just works like it was made to be.