There are several alternatives that you could try:
Method 1:
Create a “split” at the start position (“Ctrl + i”)
(See: Edit Menu: Audio Clips - Audacity Manual)
Method 2:
Add a “label” at the start position (“Ctrl + B” then “Enter”)
(See: Label Tracks - Audacity Manual)
Method 3:
Start playback from the start position. When the “end” position is reached, while the audio is still playing, press “]”.
(See: Select Menu: Region - Audacity Manual)
Method 4:
Zoom out (Zooming Overview - Audacity Manual).
Make an approximate selection by eye.
Zoom in on one end of the selection and adjust it more precisely with “Shift + Click and drag”.
Zoom in on the other end of the selection and adjust as necessary.
The quickest way to find the peak level of a selection is to open the “Amplify” effect. The default “Amplification (dB)” amount is the required amplification to bring the selection to 0 dB. Thus, if the Amplify effect says “3.5 dB”, then you know that the current level is “-3.5 dB”. Cancel the effect after you have looked.
A quick way to find the RMS level of a selection is with the “Contrast” tools. See: Contrast - Audacity Manual
There is also an optional plug-in called “Wavestats”, available here: Wavestats.ny plug-in
Unfortunately this is only available in English, but it could be modified to support other languages. “Wave Stats” shows a lot of information about the selected audio:
