Hi,
Maximum chars
Yes, you can use Text field type and set a custom format, for example for a maximum chars you can use regex like

.{0,20}

see an example in screenshot

With Regex you can specify also which chars are allowed. Of course if you do not have knowledge about Regular Expressions then you need to read some docs in internet (if your requirement is simple then we can help you )

Message (Tip)
There are 2 different ways to add description messages,
1. Tooltip on mouse hover: you can fill html editor in Edit field mode
2. Static text in a field position: you can create a field of type HTML Delimiter (docs at http://docs.easy-profile.com/index.php/article/html-delimiter-field-type)

Thanks! Is solved with your answer. So I have one more question, Can I put a message when someone exceeds the character limit in a field type textarea?

Hi,
sorry for late reply, it was night for us.

Sorry this is not possible.
The best way is to add some javascript code to manage this (maybe with a chars counter).

thanks! for everything!. And the last question, How can I show larger profile image in a profile view?

You can manage this with simple CSS, try to add for example this code in your template style:

.jsn-p-avatar img{

    width:200px /* Width of profile image */

}

.jsn-p-top-a .jsn-p-title, .jsn-p-top-a .jsn-p-before-fields {

    margin-left: 220px; /* Margin of profile header info, basically ImageWidth+20 */

}

I have added, but I´m not sure if I added it in correct file. I put in the file style.css from folder \components\com_jsn\assets\css but doesn't work

Please do not add this into component folder, you will lost your changes on component upgrade, Styles should be managed only from Template.

Many templates have a setting to add custom code or a custom CSS file ( normally at /templates/your-template-folder/css/custom.css )