1a. Concerning the Designer, you will find recommendations in the README.txt file. Some more information: installation needs about 300Mb disk space, and a development workspace may need between 10Mb and +500Mb (depending on your developments). CPU is generally not an issue, most machines from the past few years are OK.
1b. The Runtime can work with 512Mb RAM, but the sizing really depends on what sort of process you will execute (monitoring the java processes can help). Usually customers set it between 512Mb and 2048Mb. Installation needs 100Mb disk space, + another 100Mb for log files (depends on the Runtime's activity but should be a good start). The Log Database can become pretty big if you do not setup session purges. Depending on the purges you setup and the complexity/verbosity of your processes, it can be between 200Mb and 10Gb (or more ?)...
Stambia lets you develop processes of different types and complexity. There is no pre-defined rule for sizing. So really the best practice is to observe your JVM's behavior as soon as you start testing your developments, and adjust your JVM accordingly.
2. Menu Help / About Stambia / License manager / Enter new license key => paste into the Server field.