Yes, the top and bottom lines can be customized.
Top line can have max of 23 characters including spaces.
Bottom line can fit more character at a max of 32 characters including spaces.
Make sure to put the below info in the note to seller box during etsy checkout if you are ordering gift tags.
Top line:_______
Bottom line:_________

Most washi tape will have the measurement in the item detail. Some of the conversions are as follows:
Usually the width:
15mm = 0.590551 inch
Usually the length:
10m = 32.8084 ft = 393.701 inches = 10.9361 yards
15m = 49.2126 ft = 590.551 inches = 16.4042 yards

If you have other metric conversion needed, it's easiest to type in the google search for example "12in to yard" "12mm to ft" etc and google will show the conversion.

Processing time can vary and most items such as washi tape will be about 1-3 business days (not counting weekend)

Most custom items can take 3-5 business days (not counting weekends).

First class domestic within the US is about 4-10 business days (not include processing time).

If you need faster domestic shipment, you can upgrade with the below link (ONLY FOR US)
Priority - 2-4 business days but will depend on USPS to be on schedule.
Express - 1-2 business days and usually treated by USPS as more of a priority.
https://www.etsy.com/listing/226431239/reserved

First class international is approximately 9-15 business days (not include processing time), so it can vary depending on the country. Keep in mind international mail does need to go through each country's customs and sometimes that can cause delays especially during holidays.