Hi!
I've got several questions. Please help!
1. It there any function like SetShareSize but which operates with %% of equity but not shares quantity? I want to write a code which can buy shares at a percentage part of capital. For example 50%, 12%, 130%, 178%... depending on market conditions.
2. What kind of function returns current equity size? I need to know it to calculate a capital to use in the trades.

The answer is "There isn't." to both.

1. As to "SetShareSize for equity", there exist two solutions:

a. Simply use the Position Options PosSizer's "Use .Tag" feature which allows to assign a % Equity or Max % Risk figure programmatically in your Strategy code.

b. Or build your own programmatic solution if you want something more custom: you could program a D.I.Y. PosSizer:

+ See this thread for a code example
+ Wiki FAQ > How to size a Position based on a DataSeries value? How to limit the number of entered positions based on a DataSeries value?
+ Wiki FAQ > How to pass a value from Strategy to PosSizer for Alert sizing?

2. Here's a workaround that may work out:

WealthScript Techniques | Interacting Dynamically with Portfolio Level Equity

And how to use "Use.Tag" in Pos Sizer for changing % Equity programmatically ?
Where can I switсh on this option?

Click the link, it contains a sufficient instruction.

I've done:
1. installed MS123 PosSizer Library.
2. Chosen in PosSizer "Position Options" then Use.tag and %Equity.

How to set value to %Equity in my code?
I used from your example

CODE:
Please log in to see this code.

command LastPosition.Tag=20;

It doesn't work :(
What did I wrong?

"Doesn't work" just doesn't work here. As many other users who have tried it, I'm sure that it works.

Please read this carefully: How to Report Bugs Effectively > "It doesn't work", then follow author's suggestions and summary. Having finished that, please describe the problem in detail:

* How did you determine that?
* You aren't using this to size Alerts, right? Because for Alerts this doesn't function.
* To clarify explanations, send a screen shot of Position Options by striking the "Print Screen" key and then pasting into Paint or other imaging application. Save as PNG and attach.
* Setting (if required to reproduce): single symbol or Multi-symbol mode, position sizing / data loading settings, symbol(s) and data provider

If I use MS123 PosSizer Library and Position Options "using" what shoud I write in the begining of my code to get access to Position Options features?

For example, here there is a description of WLD indicators:
using WealthLab.Indicators;

And for Position Options
using what?

Such usage of PosSizers is not supported, so you're on your own with it.