3.3V PSU is always a challenge on small PCB’s. I tend to end up with linear regulators due to size and cost. They are are great, but regulating from 24V to 3.3V means you use more effect on the regulator than you get out. Sometimes I also need to regulate to 5V and continue to 3.3V. I seriously need to move on to switched PSU’s.
The circuit above is D24V3F3 from POLOLU giving 300mA. It’s 13 x 10mm, but it cost 8.- GBP on ebay and 5,5 USD on hobbytronics.co.uk.
This is a larger one based on LM2596 that can regulate from 1,8 to 36V with 3A or so output. The breakout board can be bought for ca 50 pence on ebay in bulk. I use these a lot because they are great and I can’t even buy the components for that price. 3A is however far more than I need, 300mA is probably ok. An MCU usually tick on 10mA, but transceivers for RS485, Wifi, Ethernet etc tend to use a bit more.
Looking at making a similar circuit myself I found LMR14208 from TI that comes in a thin 6 pin SO package and delivers 600mA. The reference schematics only use 8 components all included handling input voltage up to 42V.
This schematics are from LMR14208 datasheet and it also contains examples for 12V, 15V and 0,8V. I need to find a diode and coil, but it should be very doable to implement a Switched PSU using the same space as the POLOLU one with 600 rather than 300mA. LMR14206 is sadly priced around 1.- USD from the Sources I have, but well – can’t always win.